1873.] 191 [McCrady. 



in no way does it tend to invalidate the existence of these four 

 groups. 



If, for example, we symbolise the Animal Kingdom by a great 

 tree, of which the short but common trunk is, roughly speaking, the 

 egg, or the earth, we may conceive in accordance with our present 

 knowledge, this trunk as bifurcating into two great stems or sub- 

 ordinate trunks, diverging in opposite directions, each of which again 

 divides into two great branches. Then we observe that though these 

 two branches on each great stem are usually very distinct in their foli- 

 age, and that one stands on the whole altogether lower than the 

 other, yet that their outmost twigs often so intermingle as to render it 

 difficult to decide to which branch each belongs, and that actually we 

 find the lower branchlets of the higher branch sometimes to hang 

 lower than the highest branchlets of the lower. Nay, further, that 

 in spite of the opposite divergence between the two main stems, 

 there is an important, though disguised parallelism between their sec- 

 ondary branches, and that not unfrequently an aberrant branchlet 

 from one stem appears to us as if it might possibly belong to the 

 other, until embryology has traced it to its source. 



By the important parallelism referred to, I mean the fact that the 

 Radiata, the lower branch of one stem, and the Mollusca, the lower 

 branch of the other, correspond with each other in the sacciform 

 structure of their bodies, though they differ in the type and disposi- 

 tion of their organs; while at the same time the Articulata, the higher 

 branch of the one stem, and the Vertebrata, the higher branch of the 

 other, though differing in the type and disposition of organs even to 

 direct opposition, so as to be antitropes, actually agree in the circum- 

 stance that in each case the body is arthro-cylindrical. And this 

 parallelism involves a Paralogy or whole series of analogies between 

 the two great stems. 



Nor does the recognition of these differences in the least obscure 

 the fact that the tree of life is one tree, the animal kingdom a unit, 

 having for its origin the earth, which either was itself, or else con- 

 tained the germ whence all organic types have originated, probably 

 by fissiparition, in a manner analogous to the gemmation of polymor- 

 phic communities from a common embryonic stock, such as we see in 

 Siphonophorse, and many Polyzoa; but distinguished from such cases 

 by the circumstance that the forms produced are all detached (in- 

 stead of attached) parasites of their embryonic source, and also be- 

 cause their fissiparition must have been preceded by a peculiar differ- 



