452 THE EVOLUTION OP LIFE. 



stage, every organism has the greatest number of characters 

 in common with all other organisms in their earliest stages; 

 that at a stage somewhat later, its structure is like the struc- 

 tures displayed at corresponding phases by a less extensive 

 multitude of organisms; that at each subsequent stage, 

 traits are acquired which successively distinguished the de- 

 veloping embryo from groups of embryos that it previously 

 resembled thus step by step diminishing the class of 

 embryos which it still resembles; and that thus the class of 

 similar forms is finally narrowed to the species of which it is 

 a member." 



Assuming for a moment that this generalization is true as 

 it stands, or rather, assuming that the qualifications needed 

 are not such as destroy its correspondence with the average 

 facts, we shall see that it has profound significance. For if 

 we follow out in thought the implications if we conceive 

 the germs of all kinds of organisms simultaneously develop- 

 ing, and imagine that after taking their first step together, 

 at the second step one half of the vast multitude diverges 

 from the other half; if, at the next step, we mentally watch 

 the parts of each great assemblage beginning to take two or 

 more routes of development; if we represent to ourselves 

 such bifurcations going on, stage after stage, in all the 

 branches; we shall see that there must result an aggregate 

 analogous, in its arrangement of parts, to a tree. If this vast 

 genealogical tree be contemplated as a whole, made up of 

 trunk, main branches, secondary branches, and so on as far 

 as the terminal twigs; it will be perceived that all the 

 various kinds of organisms represented by these terminal 

 twigs, forming the periphery of the tree, will stand related 

 to one another in small groups, which are united into groups 

 of groups, and so on. The embryological tree, expressing the 

 developmental relations of organisms, will be similar to the 

 tree which symbolizes their classificatory relations. That 

 subordination of classes, orders, genera, and species, to which 

 naturalists have been gradually led, is just that subordination 



