SUMMARY. 163 



mals had a common history up to the time of the 

 appearance of the first multicellular animal, i. e. y the 

 first animal in which the cells had ceased to be in- 

 dependent, and had formed a unity, the various 

 parts having different functions. From this point 

 there is a divergence in many directions. And here 

 the embryological evidence is lacking, for we can- 

 not yet tell to how many lines of descendants this 

 common ancestor gave rise, nor what were the sort of 

 changes which converted this form into the sub- 

 kingdoms which came from it. There is, then, at 

 this point, a gap which, as yet, cannot be filled with 

 any reliable stages. But the existence of this gap 

 does not detract from the all-important conclusion 

 that, by means of embryological evidence, it is pos- 

 sible to demonstrate a common starting-point of 

 even the most diverse branches of the animal king- 

 dom. 



A second method by which embryology has as- 

 sisted in bringing about an understanding of the 

 animal kingdom is, by enabling the naturalist to 

 discover the direction of development taken by the 

 various orders within the types ; or, in other words, 

 to discover the relation of the different divisions of 

 the various sub-kingdoms to each other. In this 

 task collateral evidence can be obtained for some 

 groups from fossils. The amount of evidence of all 

 kinds is constantly increasing, and although, as yet, 

 it is quite insufficient for the final solution of many 

 questions, the general tendency is all in the s,ame 

 direction. Leaving the gastrula and passing the 

 gap we have mentioned, the next step is the appear- 



