THE EVIDENCE FROM DEVELOPMENT. 



221 



FIG. 137. SNAIL. 



Thus much for what is observed in the development of animals, 

 as already illustrated in these pages. What is to be inferred by 

 the biologist from the facts of early development? The reply 

 was clearly enough given in the phrase, "development repeats 

 descent;" or, otherwise, "the history of an individual's development 

 presents us with a panoramic or changing picture, more or less 

 obscured, of the descent 

 or development of its 

 race." In the absence of 

 such a thought, all de- 

 velopment is a mystery. 

 Rejecting the idea that 

 the phases of individual 

 development repeat the 

 evolution of the species, 

 we may only say that the 

 facts of natural history are 

 either each a senseless 

 paradox, or "form a mere 

 snare to entrap our judgment." Even in the later developments of 

 animals, we were able to trace, as we have seen, striking likenesses, 

 provable only on the theory of evolution. The mere reference to 

 Crustaceans and Echinoderms, will suffice to indicate the grounds 

 on which the latter assertion is based ; whilst the history of the 

 insect-class in its developmental 

 aspect will shortly be shown 

 to teach the same practical and 

 pregnant lesson. It might be 

 thought that the teachings of 

 development had by these 

 examples received copious 

 enough illustration. But there 

 remain for notice one or two 

 life-histories which, whilst they 

 may trench upon fields already 

 treated, possess yet an interest 

 of their own. It is to these 

 latter examples that we now refer by way of a closing reference to 

 the early history of animals at large. 



Above the rank of the insects, or at least in a different group of 

 the animal world from that in which they are contained, we may find 

 plain illustration of that connection between apparently different 

 classes of animals which evolution explains in rational and con- 

 sistent fashion. The group of the Mollusca, known popularly as 

 that of the " shell-fish," and having as its typical members the oysters, 



FIG. 138. SLUGS. 



