ANIMAL EVOLUTION 9 



of cells that are apparently alike, but it is obvious 

 that they are really unlike, for when a certain number — 

 a hundred and twelve it may be — are present, they form 

 groups, and these groups undergo changes of position 

 resulting in the formation of layers, the cells composing 

 each layer being generally distinguishable by their form. 

 In each layer fresh groups arise, their cells again exhibiting 

 differences of form ; and these groups undergo further 

 re-arrangement, and so the process goes on until finally, 

 as a result of the regrouping and differentiation of the 

 cells, all derived from the original germ-cell, we get the 

 specific organs arranged in the specific order characteristic 

 of the adult organism. 



Visibly, then, every individual animal and every plant 

 goes through its own course of evolution. Its life-history 

 is a becoming of the complex and heterogeneous out 

 of the simple and homogeneous. Thus there is a 

 parallelism between the evolution of any given individual 

 .organism, and the evolution of the whole assemblage of 

 organisms that exist or ever have existed ; more particu- 

 larly a parallelism between the evolution of any given 

 organism and the evolution of the race or kind to which 

 it belongs. 



But there is a great difference. 



If, in imagination, we trace the genealogy of any 

 animal back through the ages, we see that, in the begin- 

 ning, it must have had its origin in real simplicity and 

 homogeneity. All that is now complex and hetero- 

 geneous in any organism must have been added, bit by 

 bit, part by part, as the result of countless interactions 

 and reactions always in operation through the long 

 course of time. 



We cannot conceive, and we do not suppose, that the 

 specific complexity of any given animal was present, 

 in any sense, in the primordial ancestor from which it is 

 descended. It is only a mystification to say, as some 

 have said, that it was potentially present, since proto- 



B 



