172 THE INDUCTIONS OF BIOLOGY. 



The order of Nature is habitually more complex than our 

 generalizations represent it as being refuses to be fully 

 expressed in simple formulae; and we are obliged to limit 

 them by various qualifications. It is thus here. Since von 

 Baer's day the careful observations of numerous observers 

 have shown his allegation to be but approximately true. 

 Hereafter, when discussing the embryological evidence of 

 Evolution, the causes of deviations will be discussed. For 

 the present it suffices to recognize as unquestionable the fact 

 that whereas the germs of organisms are extremely similar, 

 they gradually diverge widely, in modes now regular and 

 now irregular, until in place of a multitude of forms practi- 

 cally alike we finally have a multitude of forms most of 

 which are extremely unlike. Thus, in conformity with the 

 law of evolution, not only do the parts of each organism 

 advance from indefinite homogeneity to definite hetero- 

 geneity, but the assemblage of all organisms does the same: 

 a truth already indicated in First Principles. 



53. This comparison between the course of development 

 in any creature, and the course of development in all other 

 creatures this arrival at the conclusion that the course of 

 development in each, at first the same as in all others, be- 

 comes stage by stage differentiated from the courses in all 

 others, brings us within view of an allied conclusion. If we 

 contemplate the successive stages passed through by any 

 higher organism, and observe the relation between it and its 

 environment at each of these stages; we shall sec that this 

 relation is modified in a way analogous to that in which the 

 relation between the organism and its environment is modi- 

 fied, as we advance from the lowest to the highest grades. 

 Along with the progressing differentiation of each organism 

 from others, we find a progressing differentiation of it from 

 its environment; like that progressing differentiation from 

 the environment which we meet with in the ascending forms 

 of life. Let us first glance at the way in which the ascend- 



