l exposed to certain conditions during several generations are modified in the same manner。〃 Indefinite variation is much more general and a more important factor in the production of new species； as a result of this； single individuals are distinguished from one another by 〃slight〃 differences； first in one then in another character。 There may also occur； though this is very rare； more marked modifications； 〃variations which seem to us in our ignorance to arise spontaneously。〃 （〃Origin of Species〃 （6th edition）； page 421。） The selection theory demands the further postulate that such changes； 〃whether extremely slight or strongly marked；〃 are inherited。 Darwin was no nearer to an experimental proof of this assumption than to the discovery of the actual cause of variability。 It was not until the later years of his life that Darwin was occupied with the 〃perplexing problem。。。what causes almost every cultivated plant to vary〃 （〃Life and Letters〃； Vol。 III。 page 342。）： he began to make experiments on the influence of the soil； but these were soon given up。
In the course of the violent controversy which was the outcome of Darwin's work the fundamental principles of his teaching were not advanced by any decisive observations。 Among the supporters and opponents； Nageli （Nageli； 〃Theorie der Abstammungslehre〃； Munich； 1884； cf。 Chapter III。） was one of the few who sought to obtain proofs by experimental methods。 His extensive cultural experiments with alpine Hieracia led him to form the opinion that the changes which are induced by an alteration in the food…supply； in climate or in habitat； are not inherited and are therefore of no importance from the point of view of the production of species。 And yet Nageli did attribute an important influence to the external world； he believed that adaptations of plants arise as reactions to continuous stimuli； which supply a need and are therefore useful。 These opinions； which recall the teleological aspect of Lamarckism； are entirely unsupported by proof。 While other far…reaching attempts at an explanation of the theory of descent were formulated both in Nageli's time and afterwards； some in support of； others in opposition to Darwin； the necessity of investigating； from different standpoints； the underlying causes； variability and heredity； was more and more realised。 To this category belong the statistical investigations undertaken by Quetelet and Galton； the researches into hybridisation； to which an impetus was given by the re… discovery of the Mendelian law of segregation； as also by the culture experiments on mutating species following the work of de Vries； and lastly the consideration of the question how far variation and heredity are governed by external influences。 These latter problems； which are concerned in general with the causes of form…production and form… modification； may be treated in a short summary which falls under two heads； one having reference to the conditions of form…production in single species； the other being concerned with the conditions governing the transformation of species。
I。 THE INFLUENCE OF EXTERNAL CONDITIONS ON FORM…PRODUCTION IN SINGLE SPECIES。
The members of plants； which we express by the terms stem； leaf； flower； etc。 are capable of modification within certain limits； since Lamarck's time this power of modification has been brought more or less into relation with the environment。 We are concerned not only with the question of experimental demonstration of this relationship； but； more generally； with an examination of the origin of forms； the sequences of stages in development that are governed by recognisable causes。 We have to consider the general problem； to study the conditions of all typical as well as of atypic forms； in other words； to found a physiology of form。
If we survey the endless variety of plant…forms and consider the highly complex and still little known processes in the interior of cells； and if we remember that the whole of this branch of investigation came into existence only a few decades ago； we are able to grasp the fact that a satisfactory explanation of the factors determining form cannot be discovered all at once。 The goal is still far away。 We are not concerned now with the controversial question； whether； on the whole； the fundamental processes in the development of form can be recognised by physiological means。 A belief in the possibility of this can in any case do no harm。 What we may and must attempt is thisto discover points of attack on one side or another； which may enable us by means of experimental methods to come into closer touch with these elusive and difficult problems。 While we are forced to admit that there is at present much that is insoluble there remains an inexhaustible supply of problems capable of solution。
The object of our investigations is the species； but as regards the question； what is a species； science of to…day takes up a position different from that of Darwin。 For him it was the Linnean species which illustrates variation： we now know； thanks to the work of Jordan； de Bary； and particularly to that of de Vries （de Vries； 〃Die Mutationstheorie〃； Leipzig； 1901； Vol。 I。 page 33。）； that the Linnean species consists of a large or small number of entities； elementary species。 In experimental investigation it is essential that observations be made on a pure species； or； as Johannsen （Johannsen； 〃Ueber Erblichkeit in Populationen und reinen Linien〃； Jena； 1903。） says； on a pure 〃line。〃 What has long been recognised as necessary in the investigation of fungi； bacteria and algae must also be insisted on in the case of flowering plants； we must start with a single individual which is reproduced vegetatively or by strict self…fertilisation。 In dioecious plants we must aim at the reproduction of brothers and sisters。
We may at the outset take it for granted that a pure species remains the same under similar external conditions； it varies as these vary。 IT IS CHARACTERISTIC OF A SPECIES THAT IT ALWAYS EXHIBITS A CONSTANT RELATION TO A PARTICULAR ENVIRONMENT。 In the case of two different species； e。g。 the hay and anthrax bacilli or two varieties of Campanula with blue and white flowers respectively； a similar environment produces a constant difference。 The cause of this is a mystery。
According to the modern standpoint； the living cell is a complex chemico… physical system which is regarded as a dynamical system of equilibrium； a conception suggested by Herbert Spencer and which has acquired a constantly increasing importance in the light of modern developments in physical chemistry。 The various chemical compounds； proteids； carbohydrates； fats； the whole series of different ferments； etc。 occur in the cell in a definite physical arrangement。 The two systems of two species must as a matter of fact possess a constant difference； which it is necessary to define by a special term。 We say； therefore； that the SPECIFIC STRUCTURE is different。
By way of illustrating this provisionally； we may assume that the proteids of the two species possess a constant chemical difference。 This conception of specific structure is specially important in its bearing on a further treatment of the subject。 In the original cell； eventually also in every cell of a plant； the characters which afterwards become apparent must exist somewhere； they are integral parts of the capabilities or potentialities of specific structure。 Thus not only the characters which are exhibited under ordinary conditions in nature； but also many others which become apparent only under special conditions （In this connection I leave out of account； as before； the idea of material carriers of heredity which since the publication of Darwin's Pangenesis hypothesis has been frequently suggested。 See my remarks in 〃Variationen der Bluten〃； 〃Pringsheim's Jahrb。 Wiss。 Bot。〃 1905； page 298； also Detto； 〃Biol。 Centralbl。〃 1907； page 81； 〃Die Erklarbarkeit der Ontogenese durch materielle Anlagen〃。）； are to be included as such potentialities in cells； the conception of specific structure includes the WHOLE OF THE POTENTIALITIES OF A SPECIES； specific structure comprises that which we must always assume without being able to explain it。
A relatively simple substance； such as oxalate of lime； is known under a great number of different crystalline forms belonging to different systems （Compare Kohl's work on 〃Anatomisch…phys。 Untersuchungen uber Kalksalze〃； etc。 Marburg； 1889。）； these may occur as single crystals； concretions or as concentric sphaerites。 The power to assume this variety of form is in some way inherent in the molecular structure； though we cannot； even in this case； explain the necessary connection between structure and crystalline form。 These potentialities can only become operative under the influence of external conditions； their stimulation into activity depends on the degree of concentration of the various solutions； on the nature of the particular calcium salt； on the acid or alkaline reactions。 Broadly speaking； the plant cell behaves in a similar way。 The manifestation of each form； which is inherent as a potentiality in the specific structure； is ultimately to be referred to external conditions。
An insight into this connection is； however； rendered exceedingly difficult； often quite impossible； because the environment never directly calls into action the potentialities。 Its influence is exerted on what we may call the inner world of the organism； the importance of which increases with the degree of differentiation。 The production of form in every plant depends upon processes in the interior of the cells； and the nature of these determines which among the possible characters is to be brought to light。 In no single case are we acquainted with the internal process responsible for the production of a particular form。 All possible factors may play a part； such as osmotic pressure； permeability of the protoplasm； the degree of concentration of the various chemical substances； etc。； all these factors should be included in the category of INTERNAL CONDITIONS。 This inner world appears the more hidden from our ken because it is always represented by a certain definite state； whether we are dealing with a sing