MONISTIC EVOLUTION 131 



various properties of the animals which spring from 

 them. As we trace them onward in their develop 

 ment, we see these differences manifesting themselves. 

 At first all pass, according to Haeckel, through a 

 stage which he calls the gastrula, in which the whole 

 body is represented by a sort of sac, the cavity of 

 which is the stomach, and the walls consist of two 

 layers of cells. It should be stated, however, that 

 many eminent naturalists dissent from this view, and 

 maintain that even in the earliest stages material 

 differences can be observed. In this they are pro 

 bably right, as even Haeckel has to admit some 

 degree of divergence from this all-embracing 

 gastraea theory. Admitting, however, that such 

 early similarity exists within certain limits, we find 

 as the embryo advances that it speedily begins to 

 indicate whether it is to be a coral animal, a snail, a 

 worm, or a fish. Consequently the physiologist who 

 wishes to trace the resemblances leading to mammals 

 and to man has to lop off, one by one, the several 

 branches which lead in other directions, and to follow 

 that which conducts by the most direct course to the 

 type which he has in view. In this way Haeckel can 

 show that the embryo Homo sapiens is in successive 

 stages so like to the young of the fish, the reptile 

 the bird, and the ordinary quadruped that he can 

 produce for comparison figures in which the cursory 

 observer can detect scarcely any difference. 



All this has long been known, and has been re- 



I 2 



