January 17, 1908] 



SCIENCE 



91 



tion of the egg cell from that of any tissue 

 cell is the fact that the latter gives rise 

 only to a particular type of cell, with the 

 formation of which differentiation comes 

 to an end, whereas the former undergoes 

 a long series of differentiations and gives 

 rise to a complicated organism. 



II 



The causes of differentiation, and hence 

 of heredity, are in general twofold, in- 

 trinsic and extriusic ; the former are repre- 

 sented by the germinal protoplasm, the 

 latter by practically all other conditions. 

 Similarity of differentiation in successive 

 generations, or hereditary likeness depends 

 upon similarity of both the intrinsic and 

 the extrinsic factors. The differentiations 

 of the germ folloAv a definite sequence be- 

 cause the substance of the germ undergoes 

 definite chemical transformations, which 

 are predetermined by its initial constitu- 

 tion. Similarity of sequence is involved in 

 similarity of germinal substance and of 

 environment. 



There are no vital structures or func- 

 tions which are absolutely independent, 

 self-acting, self-moving, self-differentiating 

 or independently variable. Each part 

 and function exists only in close rela- 

 tionship with other parts and functions 

 and with environmental conditions. Many 

 of the criticisms which have been recently 

 brought against "unit characters," "units 

 of heredity, " " organ-forming substances, ' ' 

 "individuality of the chromosomes," in- 

 deed against heredity and variation as a 

 whole, are applicable only to extreme views, 

 which no one consciously holds. To be 

 sure, an individual, whether a unit quality, 

 a chromosome, a substance or a, person, can 

 not exist apart from its environment, but 

 who has ever maintained the contrary 1 



A study of the phenomena of develop- 

 ment, no less than the principle that every 

 effect must have an adequate cause, makes 



it certain that the characteristics of an 

 organism are in some way predetermined 

 within the protoplasm of the fertilized egg 

 cell. From a frog's egg only a frog will 

 develop, from an echinoderm's egg only an 

 echinoderm, and the course of development 

 is, under constant external conditions, 

 marked out in each case, even down to the 

 minutest details. Since, however, these ex- 

 ternal conditions may be exactly the same 

 in the case of two eggs, and yet the results 

 of development be very different in the 

 two, we can only conclude that the physical 

 basis of inheritance is to be found in the 

 properties of the germinal protoplasm. To 

 assume that extrinsic causes determine 

 whether there shall hatch from an egg a 

 chicken or an eagle would be the sheerest 

 nonsense. The fact is there is no escape 

 from the conclusion that all really in- 

 herited characteristics are predetermined in 

 the structure of the germinal protoplasm. 

 But it should be observed that to say that 

 characteristics are predetermined is a very 

 different thing from saying that they are 

 preformed. The one merely aiSrms that 

 the causes of the transformations which 

 lead from one step to another in the de- 

 velopment are determined by the initial 

 constitution of the fertilized egg ; the other 

 affirms that these transformations have 

 already taken place within the egg. 



Ill 



It is practically certain that in the last 

 analysis the characteristics of the germ are 

 dependent upon its chemical and physical 

 constitution. Fick has lately maintained 

 that not only the protoplasm of every 

 species, but also that of every individual, 

 must be different from that of every other. 

 At the same time he points out the fact 

 that this apparently stupendous assump- 

 tion is readily possible within the limits of 

 the composition of protoplasm, since 

 Miescher has shown that a molecule of 



