134 THE CANADIAN NATURALIST. [June 



conditions of the case, that was a correlation arising from the 

 plans of the Creator, and with which their poor brains and 

 greater or less safety and comfort had nothing to do. If we 

 were disposed to accept this hypothesis of Owen, we should not 

 ill doing so arrive at any true cause, and we should at the same 

 time find ourselves involved in the old difficulties. That a 

 Hipparion should change into a horse it would be necessary that 

 not only his feet but his teeth and other structures should 

 change in harmony with each other. This must take place 

 either at once or gradually. If at once, then a pair of horses 

 must be born from Hipparia in one herd, and must be isolated 

 from the rest so as to produce a herd of horses. This is hard to 

 believe ; and if we resort to gradual change, the required 

 isolation of the breed will be still more difficult to secure. The 

 demands upon our faith are obviously greater here than even in 

 the hypothesis of Darwin, — that is if we can be induced to place 

 any reliance on the argument of the latter as to struggle for 

 existence. 



(4.) The last of these hypotheses which I shall notice, and, in 

 my view, the most promising of them all, is one which has recently 

 been ably advocated by Mr. Edward D. Cope in a memoir on the 

 " Origin of Genera," published in the Proceedings of the 

 Academy of Natural Sciences,* and which is based on the well- 

 known analogy between embryonic changes, rank in the Zoological 

 scale and Geological succession. It may be illustrated by the 

 remarkable and somewhat startling fact, that while no authenti- 

 cated case exists of animals changing from one species to another, 

 they are known to change from one genus or family to another, 

 and this without losing their individuality. Prof. Dumeril, of 

 Paris, and Prof. Marsh, of New Haven, have recently directed 

 attention to the fact that species of Siredon, reptiles of the Lakes 

 of the Rocky Mountains and of Mexico, and which, like our 

 North American Menohranclius, retain their gills during life, 

 when kept in captivity in a warmer temperature than that which 

 is natural to them, lose their gills, and pass into a form hitherto 

 regarded as of a difiierent genus and family, — the genus 

 Amhlysoma. In this case we may either suppose that the 

 Amblysoma, under unfavourable circumstances, has its maturity 

 and reproduction prematurely induced before it has lost its 



* Philadelphia, 18G9. 



