ery form there are certain favourable conditions of nourishment； which may be defined for each species。 Experience shows that； within definite and often very wide limits； it does not depend upon the ABSOLUTE AMOUNT of the various food substances； but upon their respective degrees of concentration。 As we have already stated； the production of flowers follows a relative increase in the amount of carbohydrates formed in the presence of light； as compared with the inorganic salts on which the formation of albuminous substances depends。 （Klebs； 〃Kunstliche Metamorphosen〃； page 117。） The various modifications of flowers are due to the fact that a relatively too strong solution of salts is supplied to the rudiments of these organs。 As a general rule every plant form depends upon a certain relation between the different chemical substances in the cells and is modified by an alteration of that relation。
During long cultivation under conditions which vary in very different degrees； such as moisture； the amount of salts； light intensity； temperature； oxygen； it is possible that sudden and special disturbances in the relations of the cell substances have a directive influence on the inner organisation of the sexual cells； so that not only inconstant but also constant varieties will be formed。
Definite proof in support of this view has not yet been furnished； and we must admit that the question as to the cause of heredity remains； fundamentally； as far from solution as it was in Darwin's time。 As the result of the work of many investigators； particularly de Vries； the problem is constantly becoming clearer and more definite。 The penetration into this most difficult and therefore most interesting problem of life and the creation by experiment of new races or elementary species are no longer beyond the region of possibility。
XIV。 EXPERIMENTAL STUDY OF THE INFLUENCE OF ENVIRONMENT ON ANIMALS。
By JACQUES LOEB； M。D。 Professor of Physiology in the University of California。
I。 INTRODUCTORY REMARKS。
What the biologist calls the natural environment of an animal is from a physical point of view a rather rigid combination of definite forces。 It is obvious that by a purposeful and systematic variation of these and by the application of other forces in the laboratory； results must be obtainable which do not appear in the natural environment。 This is the reasoning underlying the modern development of the study of the effects of environment upon animal life。 It was perhaps not the least important of Darwin's services to science that the boldness of his conceptions gave to the experimental biologist courage to enter upon the attempt of controlling at will the life…phenomena of animals； and of bringing about effects which cannot be expected in Nature。
The systematic physico…chemical analysis of the effect of outside forces upon the form and reactions of animals is also our only means of unravelling the mechanism of heredity beyond the scope of the Mendelian law。 The manner in which a germ…cell can force upon the adult certain characters will not be understood until we succeed in varying and controlling hereditary characteristics； and this can only be accomplished on the basis of a systematic study of the effects of chemical and physical forces upon living matter。
Owing to limitation of space this sketch is necessarily very incomplete； and it must not be inferred that studies which are not mentioned here were considered to be of minor importance。 All the writer could hope to do was to bring together a few instances of the experimental analysis of the effect of environment； which indicate the nature and extent of our control over life…phenomena and which also have some relation to the work of Darwin。 In the selection of these instances preference is given to those problems which are not too technical for the general reader。
The forces； the influence of which we shall discuss； are in succession chemical agencies； temperature； light； and gravitation。 We shall also treat separately the effect of these forces upon form and instinctive reactions。
II。 THE EFFECTS OF CHEMICAL AGENCIES。
（a） HETEROGENEOUS HYBRIDISATION。
It was held until recently that hybridisation is not possible except between closely related species and that even among these a successful hybridisation cannot always be counted upon。 This view was well supported by experience。 It is； for instance； well known that the majority of marine animals lay their unfertilised eggs in the ocean and that the males shed their sperm also into the sea…water。 The numerical excess of the spermatozoa over the ova in the sea…water is the only guarantee that the eggs are fertilised； for the spermatozoa are carried to the eggs by chance and are not attracted by the latter。 This statement is the result of numerous experiments by various authors； and is contrary to common belief。 As a rule all or the majority of individuals of a species in a given region spawn on the same day； and when this occurs the sea…water constitutes a veritable suspension of sperm。 It has been shown by experiment that in fresh sea…water the sperm may live and retain its fertilising power for several days。 It is thus unavoidable that at certain periods more than one kind of spermatozoon is suspended in the sea…water and it is a matter of surprise that the most heterogeneous hybridisations do not constantly occur。 The reason for this becomes obvious if we bring together mature eggs and equally mature and active sperm of a different family。 When this is done no egg is； as a rule； fertilised。 The eggs of a sea…urchin can be fertilised by sperm of their own species； or； though in smaller numbers； by the sperm of other species of sea…urchins； but not by the sperm of other groups of echinoderms； e。g。 starfish； brittle…stars； holothurians or crinoids； and still less by the sperm of more distant groups of animals。 The consensus of opinion seemed to be that the spermatozoon must enter the egg through a narrow opening or canal； the so…called micropyle； and that the micropyle allowed only the spermatozoa of the same or of a closely related species to enter the egg。
It seemed to the writer that the cause of this limitation of hybridisation might be of another kind and that by a change in the constitution of the sea…water it might be possible to bring about heterogenous hybridisations； which in normal sea…water are impossible。 This assumption proved correct。 Sea…water has a faintly alkaline reaction （in terms of the physical chemist its concentration of hydroxyl ions is about （10 to the power minus six）N at Pacific Grove； California； and about （10 to the power minus 5）N at Woods Hole； Massachusetts）。 If we slightly raise the alkalinity of the sea…water by adding to it a small but definite quantity of sodium hydroxide or some other alkali； the eggs of the sea…urchin can be fertilised with the sperm of widely different groups of animals； possibly with the sperm of any marine animal which sheds it into the ocean。 In 1903 it was shown that if we add from about 0。5 to 0。8 cubic centimetre N/10 sodium hydroxide to 50 cubic centimetres of sea…water； the eggs of Strongylocentrotus purpuratus （a sea…urchin which is found on the coast of California） can be fertilised in large quantities by the sperm of various kinds of starfish； brittle… stars and holothurians； while in normal sea…water or with less sodium hydroxide not a single egg of the same female could be fertilised with the starfish sperm which proved effective in the hyper…alkaline sea…water。 The sperm of the various forms of starfish was not equally effective for these hybridisations； the sperm of Asterias ochracea and A。 capitata gave the best results； since it was possible to fertilise 50 per cent or more of the sea…urchin eggs； while the sperm of Pycnopodia and Asterina fertilised only 2 per cent of the same eggs。
Godlewski used the same method for the hybridisation of the sea…urchin eggs with the sperm of a crinoid （Antedon rosacea）。 Kupelwieser afterwards obtained results which seemed to indicate the possibility of fertilising the eggs of Strongylocentrotus with the sperm of a mollusc （Mytilus。） Recently； the writer succeeded in fertilising the eggs of Strongylocentrotus franciscanus with the sperm of a molluscChlorostoma。 This result could only be obtained in sea…water the alkalinity of which had been increased （through the addition of 0。8 cubic centimetre N/10 sodium hydroxide to 50 cubic centimetres of sea…water）。 We thus see that by increasing the alkalinity of the sea…water it is possible to effect heterogeneous hybridisations which are at present impossible in the natural environment of these animals。
It is； however； conceivable that in former periods of the earth's history such heterogeneous hybridisations were possible。 It is known that in solutions like sea…water the degree of alkalinity must increase when the amount of carbon…dioxide in the atmosphere is diminished。 If it be true； as Arrhenius assumes； that the Ice age was caused or preceded by a diminution in the amount of carbon…dioxide in the air； such a diminution must also have resulted in an increase of the alkalinity of the sea…water； and one result of such an increase must have been to render possible heterogeneous hybridisations in the ocean which in the present state of alkalinity are practically excluded。
But granted that such hybridisations were possible； would they have influenced the character of the fauna？ In other words； are the hybrids between sea…urchin and starfish； or better still； between sea…urchin and mollusc； capable of development； and if so； what is their character？ The first experiment made it appear doubtful whether these heterogeneous hybrids could live。 The sea…urchin eggs which were fertilised in the laboratory by the spermatozoa of the starfish； as a rule； died earlier than those of the pure breeds。 But more recent results indicate that this was due merely to deficiencies in the technique of the earlier experiments。 The writer has recently obtained hybrid larvae between the sea…urchin egg and the sperm of a mollusc （Chlorostoma） which； in the laboratory； developed as well and lived as long as the pure breeds of the sea…urchin； and there was no