ANIMAL EVOLUTION 27 



giving rise to entire larvae). But in all cases segregation 

 sets in sooner or later. 



If I had the time, and had a great number of diagrams 

 to illustrate my argument, I could prove to you that in 

 any one class of animals such as the Chaetopod worms 

 or the Mollusca, the cell-lineages of different species, 

 though very similar, are never exactly the same. The 

 differences — which may be considerable — are mostly due 

 to the different periods at which special cells, containing 

 particular constituents of the future animal, are formed 

 in the course of segmentation. And I could further 

 show, what I can only now inform you of, that in each 

 class, and in the sub-divisions of each class, there are 

 clear indications that there is a definite relation between 

 the elaboration or simplification of the ontogeny and the 

 period at which the segregation of the specific substances 

 is accomplished in the course of segmentation. 



The life-history of an animal may be direct or indirect. 



When it is direct, the course of development is straight- 

 forward ; the animal is hatched out or born with the 

 characters proper to its species. When it is indirect, the 

 animal is born with characters very different from those 

 which it will ultimately assume. It is born as a larva, 

 and a typical larva is an independent organism, having 

 means of locomotion, a mouth, and a digestive tract. 

 It feeds itself, lays up a store of nutriment, and only 

 after a longer or shorter period of independent existence 

 does it undergo further developmental changes which 

 lead to the assumption of the adult characters. The 

 caterpillar (larva) and the butterfly (adult or imago) is 

 a familiar example of an indirect course of development. 



An indirect course of development is, in a relative sense, 

 prolonged, because there are two free stages and two 

 developmental stages leading up to them to be provided 

 for. If you could suppress the caterpillar stage, the 

 condition of the perfect insect would be arrived at sooner. 

 The development would be abbreviated. 



