120 HYPOTHESIS OF THE DEVELOPMENT OP 



mammalia. This is another advance in the scale, but more 

 remains yet to be done. The complication of the organ in- 

 creases; cavities termed ventricles are formed, which do not 

 exist in fishes, reptiles, or birds ; curiously organized parts, 

 such as the corpora striata, are added ; it is now the brain of 

 the mammalia. Its last and final change alone seems want- 

 ing, that which shall render it the brain of MAN. ( 5G ) And 

 this change in time takes place. 



So also with the heart. This organ, in the mammalia, con- 

 sists of four cavities, but in the reptiles of only three, and in 

 fishes of two only, while in the articulated animals it is 

 merely a prolonged tube. Now in the mammal foetus, at a 

 certain early stage, the organ has the form of a prolonged 

 tube ; and a human being may be said to have then the heart 

 of an insect. Subsequently, it is shortened and widened, and 

 becomes divided by a contraction into two parts, a ventricle 

 and an auricle ; it is now the heart of a fish. A subdivision 

 of the auricle afterwards makes a triple-chambered form, as 

 in the heart of the reptile tribes ; lastly, the ventricle being 

 also subdivided, it becomes a full mammal heart. 



We have now to remember that, corresponding generally 

 to these progressive forms in the development of individuals, 

 has been the succession of animal forms in the course of time. 

 Our earth bore crinoidea before it bore the higher echinoder- 

 mata. It presented crinoidea, annelides, and mollusca,, before 

 it bore fishes, and when fishes came, the first forms were 

 those cartilaginous types which correspond with the early 

 fo3tal condition of higher orders. Afterwards there were 

 reptiles, then mammifers, and finally, as we know, came man. 

 Was it, then, too much to say that, when we learned the 

 facts of embryonic development, we should see something 

 more than analogy between the progress of species upon the 

 earth and the production of an individual organism ? 



The tendency of all the illustrations is undoubtedly to 

 make us look to development as the principle which has been 

 immediately and mainly concerned in the peopling of this 

 globe, a process extending over a vast space of time, but which 



