I A CHAPTER IN DARWINISM 17 



we are entitled to conclude that all the animals which 

 have a backbone and pharyngeal gill-slits combined — 

 the Vertebrates, as we call them — have descended 

 from a common parent ; that all the animals with a 

 muscular foot-like belly and lateral gill filaments, the 

 Molluscs, have also had a common parent, and so on. 



A classification according to structure goes then a 

 long way towards mapping out the main lines of the 

 family-tree of organisms. We are further assisted in 

 the task by the fossil remains of extinct organisms 

 which sometimes give to us the actual ancestors of 

 forms now living. But the most remarkable aid to 

 the correct building up of the pedigree of animals at 

 least (and the remarks which follow are confined to 

 that division of the organic world) is afi'orded by the 

 changes — the phases of development — which every 

 animal exhibits in passing from the small shapeless 

 egg to the adult condition. The aid which we here 

 obtain depends on the following facts. Just as we 

 suppose any one animal — say a dog — to have developed 

 by slow change through an immense series of ancestors 

 which become simpler and simpler as we recede into 

 the past until we reach a small shapeless lump of living 

 matter devoid of structure, so do we find actually as 

 a matter of fact, which any one can see for himself, 

 that every individual animal begins its individual life 

 as a structureless particle which is thrown off from its 

 parent, and is known as the egg-cell (Fig. 1). Gradu- 

 ally passing through a series of more and more ela- 

 borated conditions of structure, that egg grows into 



c 



