144 THE INDUCTIONS OF BIOLOGY. 



one of the diverging groups just described, each, kind of 

 organism, though having a general direction of development 

 like that of the others it is for a time travelling with, shows 

 from the first a tendency to leave the general route — a tend- 

 ency which presently becomes strongly marked. Making 

 all requisite qualifications, however, these resemblances re- 

 main conspicuous ; and the fact that they follow each other 

 in the way described, is a fact of great significance. 



§ 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 of all 

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

 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 see that this re- 

 lation 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 

 ascending forms of life exhibit this progressing differentiation 

 from the environment. 



In the first place, it is illustrated in structure. Ad' 

 vance from the homogeneous to the heterogeneous, itself in- 

 volves an increasing distinction from the inorganic world. 

 In the lowest Protozoa we have a simplicity approaching 

 to that of air, water, or earth ; and the ascent to organisms 

 of greater and greater complexity of structure, is an ascent to 

 organisms that are in that respect more strongly contrasted 

 with the structureless environment. In form, again, 



