THE EVIDENCE FROM DEVELOPMENT. 



187 



be said to assume the special features of the group to which it 

 belongs. At this point the vertebrate will pass towards its own 

 sub-kingdom, and develop in due time the special features of the 

 fish, the frog, the reptile, the bird, or the quadruped. Hence, as 

 from a common point whence numerous ways and paths diverge, 

 each organism will elaborate or develop its " gastrula " germ into a 

 frame more or less complicated, and into belongings and structures 

 suiting its rank in the great kingdom of animal life. From such 

 a standpoint we may discern, more clearly perhaps than at any 

 other stage of our researches, the justice of the comparison which 

 symbolises the animal world and its origin by the figure of a tree, 

 whose divergent branches bear at their extremities the apparently 

 distinct and specialised groups of animals, but whose stem and trunk 

 from which these branches spring, no less powerfully represents the 

 common origin and uniform development of its varied parts. 



That the evolutionist's case for the common origin and produc- 

 tion by descent of the forms of animal life is strengthened and sup- 

 ported by the facts of development, is a statement which can admit 

 of no question in the eyes of those who fairly face the facts, and who 

 logically, and without bias or prepossession,, construe their meaning. 

 On any other supposition than that of the common origin and subse- 

 quent specialisation of the varied forms of animal life r the fact that a 

 sponge, a sea-squirt, and a lancelet pass through essentially similar 

 stages of development, pre- 

 sents itself simply as an 

 inexplicable mystery. Com- 

 munity of development be- 

 tokens community of origin ; 

 otherwise the facts of nature 

 must present themselves as 

 absurdities admitting of no 

 logical construction whatever. 

 Why a vertebrate animal in 

 its earlier history should re- 

 semble a sponge or sea-squirt 

 is a query simply unanswer- 

 able, save on the hypothesis 

 that vertebrate ancestry was 

 at one period transmitted 

 through forms of which the 

 sponges and sea-squirts are the existent and it may be the altered 

 representatives. The development of an animal thus reasonably 

 stands before us, in the newer interpretation of evolution, as a veri- 

 table panorama of its descent. Often, according to Darwin's already 

 quoted remark, the series of pictures may here and there be obscured. 



FIG. 103. DEVELOPMENT OF A FISH. 



