104 HYPOTHESIS OF THE DEVELUFME.NT O* 



and mammalia. This is another advance in the scai e, bul 

 more remains yet to be done. The complication of tha 

 organ increases; 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 wanting, that which shall render it 

 the brain of man."* And this change in time takes place. 



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

 consists 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 pro- 

 longed 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 mamma? heart. 



Another illustration here presents itself with the force 

 of the most powerful and interesting analogy. Some of 

 the earliest fishes of our globe, those of the Old Red 

 Sandstone, present, as we have seen, certain peculiarities, 

 as the one-sided tail and an inferior position of the mouth. 

 No fishes of the present day, in a mature state, are so 

 characterized ; but some, at a certain stage of their exist- 

 ence, have such peculiarities It occurred to a geologist 

 to inquire if the fish which existed before the Old Red 

 Sandstone had any peculiarities assimilating them to the 

 fcetal condition of existing fish, and particularly if they 

 were small. The first which occur before the time of the 

 Old Red Sandstone, are those described by Mr. Murchison, 

 as belonging to the Upper Ludlow Rocks ; they are all 

 rather small. Still older are those detected by Mr. Philips, 

 in the Aymestsy Limestone, being the most ancient of the 

 class which have as yet been discovered ; these are so 

 extremely minute as only to be distinguishable by the 

 microscope. Here we apparently have very clear demon - 

 strations of a parity, or rather identity, of laws presiding 



* Lord's Popular Physiology. It is to Tiedemann that we chief- 

 ly owe these curious observations- but ground was first broken 11 

 this branch of physiological science by Dr. John Hunter 



