22 



NATURE 



[September 3, 1914 



Are not the gill-slits also very different ? The changed 

 environment has had its effect. The gills themselves 

 are never developed, and the gill-slits never become 

 functional; moreover, they disappear completely at 

 later stages of development, when the conditions of 

 life become still more different and their presence 

 would be actually detrimental to their possessor. The 

 embyro with the vestigial gill-slits is, as a whole, 

 perfectly well adapted to its environment, though the 

 gill-slits themselves have ceased to be adaptive char- 

 acters. They still appear because the environrnental 

 conditions, and especially the internal conditions, 

 which have now become far more important than the 

 external ones, are still such as to cause them to do so. 

 I think the chief difficulty in forming a mental 

 picture of the manner in which evolution has taken 

 place, and especially in accounting for the phenomenon 

 of recapitulation in ontogeny, which is merely another 

 aspect of the same problem, arises from attempting 

 to take in too much at once. There is no difficulty 

 in understanding how any particular stage is related 

 to the corresponding stage in the previous genera- 

 tion, and the whole series of stages, whether^ looked 

 at from the ontogenetic or from the phylogenetic point 

 of view, can be nothing else but the sum of its suc- 

 cessive terms. 



It will be convenient, before going further, to sum 

 up the results at which we have so far arrived from 

 the point of view of the theory of heredity. We have 

 as yet seen no reason to distinguish between somato- 

 genic and blastogenic characters. All the characters 

 of the adult animal are acquired during ontogeny as 

 the result of the reaction of the organism to environ- 

 mental stimuli, both internal and external. All that 

 the organism actually inherits is a certain amount of 

 protoplasm — endowed with a certain amount of energy 

 — and a certain sequence of environmental conditions. 

 In so far as these are identical in any two successive 

 generations the final result must be identical also, the 

 child must resemble the parent ; in so far as they 

 are different the child will differ from the parent, 

 but the differences in environment cannot be ver>' great 

 without preventing development altogether. 



So far, it is clear, there has been no need to think of 

 the germ-cells as the bearers of material factors or 

 determinants that are responsible for the appearance 

 of particular characters in the adult organism ; nor 

 yet to suppose that they are, to use the phraseology 

 of the mnemic theory of heredity, charged with the 

 memories of past generations. They have been re- 

 garded as simple protoplasmic units, and the entire 

 ontogeny has appeared as the necessary result of the 

 reaction between the organism and its environment at 

 each successive stage of development. This cannot, 

 however, be a complete explanation of ontogeny, for 

 if it were we should expect all eggs, when allowed to 

 develop under the same conditions from start to finish, 

 to give rise to the same adult form, and this we know 

 is not the case. We know also, from observation 

 and experiment, that the egg is in reality by no means 

 a simple thing but an extremely complex one, and 

 that different parts of the egg may be definitely cor- 

 related with corresponding parts of the adult body. 

 It has been demonstrated in certain cases that the 

 egg contains special organ-forming substances de- 

 finitely located in the cytoplasm, and that if these 

 are removed definite parts of the organism into which 

 the egg develops will be missine. We know, also, 

 that the nucleus of the germ-cell of either sex con- 

 tains — at anv rate, at certain periods— a number of 

 perfectly well-defined bodies, the chromosomes, and 

 these also have been definitely correlated in certain 

 cases with special features of the adult organisation. 

 Before we can hope to complete our mental picture 



NO. 2340, VOL. 94] 



of the manner in which organic evolution has taken 

 place, if only in outline, it is evident that we must 

 be able to account for the great complexity of struc- 

 ture which the germ-cells themselves have managed 

 to acquire, and also to form some idea of the effect 

 of this complication upon the development of both 

 the individual and the race. , • j 



We must consider the origin of cytoplasmic and 

 nuclear complications of the egg separately, for they 

 appear to be due fundamentally to two totally distinct 

 sets of factors. In the first place we have to re- 

 member that during oogenesis the egg-cell grows to 

 a relatively large size by absorbing nutrient material 

 from the body in which it is enclosed. It is this 

 nutrient material that is used for building up the 

 deutoplasm or food-yolk. There is good reason for 

 believing that the character of this nutrient material 

 will change, during the course of evolution, ^ari passu 

 with the changing character of the organism by which 

 it is suppHed. Doubtless the change is of a chemical 

 nature, for we know from precipitin experiments that 

 the body fluids of closely allied species, or even of the 

 two sexes of the same species, do exhibit distinctly 

 recognisable differences in chemical composition. It 

 also appears highly probable, if • not certain, from 

 such experiments as those of Agar upon Simocephalus 

 that substances taken in with the food, which 

 bring about conspicuous modifications of bodily struc- 

 ture, may at the same time be absorbed and stored 

 up by the egg-cells so as to bring about corresponding 

 changes in the adults into which the eggs develop. _ 



There seems therefore to be no great difficultv in 

 comprehending, at any rate in a general way, how 

 the egg may become the repository of definite chemical 

 substances, organ-forming substances if we like to 

 call them so, possibly to be classed with the hormones 

 and enzymes, which will influence the development in 

 a particular manner as soon as the appropriate con- 

 ditions arise. r 11 • 



Unfortunately, time will not allow of our following 

 up this line of thought on the present occasion, but 

 we may notice, before passing on, that^ with the 

 accumulation of organ-forming substances in the egg 

 we have introduced the possibility of changes in 

 bodily structure, to whatever cause they may be due, 

 being represented by correlated modifications in the 

 germ-cells, and this is doubtless one of the reasons 

 why the germ-cells of different animals are not all 

 alike with regard to their potentialities of develop- 



We now come to the question of how the nucleus 

 of the germ-cell acquired its great complexity^ of 

 structure. We are not concerned here with the origin 

 of the differentiation into nucleus and cytoplasm and 

 the respective parts played by the two in the liffe of 

 the cell The problem which we have to consider is 

 the complication introduced by the sexual process, by 

 the periodicallv recurring union of the germ-cdls in 

 pairs or, as Weismann has termed it, amphimixis. 

 This 'is well known to be essentially a nuclear pheno- 

 menon, in which the so-called chromatin substance is 

 especially concerned, and it is a phenomenon which 

 must have made its appearance at a very early stage 

 of evolution, for it Is exhibited in essentially the same 

 manner alike in the higher plants and animals and 

 in unicellular organisms. 



Let us suppose, for the sake of argument, that when 

 amphimixis first took place the chromatin of each 

 germ-cell was homogeneous, but that it differed 

 sightlv in different germ cells of the same species as 

 a result of exposure to slightly different conditions 

 during Its past history-. What would be likely to 

 happen when two different samples of chromatin came 



5 Comnare Cunningham's " Hormone Theory "of Heredity ("Archiv ff.r 

 Entwicklungsmechanik der Organismen, Bd. xxvi., Helt 3y. 



