UNITY OF ORGANIZATION. 759 



557,600 species,) have been formed after one and 

 the same primitive type or model, the lower 

 orders differing from the highest chiefly by de- 

 fect, in other words, that all the animals which in- 

 habit the earth are, in reality, but one animal, as 

 maintained by Aristotle. The unity of organiza- 

 tion has been still further traced by physiologists, 

 who have found that in plants, the primitive tis- 

 sues from which all the others are evolved, con- 

 sist of nucleated cells, essentially analogous to 

 those that constitute the elementary tissue of the 

 ovum in all animals. (See Owen's Lectures on 

 Reproduction, in the Lancet of 1841.) 



And that there has been a gradually ascending 

 progress of organization, would appear from the 

 recent discoveries of geologists, who have found 

 that the oldest sedimentary formations are filled 

 with animals of the lowest class and most simple 

 structure, which becomes more and more com- 

 plex, until we arrive at the newer tertiary depo- 

 sits, which abound with the fossil remains of the 

 higher, or warm blooded species. Yet there is no 

 historical proof that fishes have ever been trans- 

 formed into reptiles, nor the latter into birds and 

 mammalia. Nor is it probable, even if such 

 were the fact, that it will ever be demonstrated 

 otherwise than by analogy, owing to the enor- 

 mous periods of time, and perhaps the great geo- 

 logical changes, requisite to bring about corres- 

 ponding changes of organization. In the present 



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