366 THE EVOLUTION OF LIFE 



N. 



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, great 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 

 each other 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 

 which results from the divergence and re-divergence of 

 embryos, as they all unfold. On the hypothesis of evolution, 

 this parallelism has a meaning indicates that primordial 

 kinship of all organisms, and that progressive differentiation 

 of them, which the hypothesis alleges. But on any other 

 hypothesis the parallelism is meaningless : or rather, it 

 raises a difficulty ; since it implies either an effect without a 

 cause, or a design without a purpose. 



129. It was said above, that this great embryological 

 law is to be taken with certain qualifications. The resem- 

 blances which hold together great groups of embryos in their 

 early stages, and which hold together smaller and smaller 

 groups in their later and later stages, are not special or 

 exact, but general or approximate ; and in some cases, the 

 conformity to this general law is very imperfect. These 

 irregularities, however, instead of being at variance with the 

 hypothesis of evolution, afford further support to it. 



Observe, first, that the only two other possible suppositions 

 respecting developmental changes, are negatived, the one by 

 this general law and the other by the minor nonconformities 

 to it. If it be said that the conditions of the case necessi- 

 tated the derivation of all organisms from simple germs, and 



