140 ESSAY ON CLASSIFICATION. 



capable of taking, and wliat a power of combination 

 must they possess, to be able to ingraft all these compli- 

 cated modes of reproduction upon structures already so 

 complicated ! But, if we turn away from mere fancies, 

 and consider the wonderful phenomena just alluded to in 

 all their bearings, how instructive they appear with refer- 

 ence to this very question of the influence of physical 

 agents upon organized beings ! For here we have animals 

 endowed with the power of multiplying in the most ex- 

 traordinary ways, every species producing new individuals 

 of its own kind differing to the utmost from their parents. 

 Does this not seem, at first, as if we had before us a perfect 

 exemplification of the manner in which different species 

 of animals may originate one from the other, and increase 

 the number of types existing at first ? And yet, with all 

 this apparent freedom of transformation, what do the 

 facts finally show 1 That all these transformations are 

 the successive terms of a cycle, as definitely closed within 

 precise limits, as in the case of animals the progeny of 

 which resembles the immediate parent in all succes- 

 sive generations. For here, as everywhere in the or- 

 ganic kingdoms, these variations are only the successive 

 expressions of a well regulated cycle ever returning to its 

 o\vn type. 



SECTION XXL 



SUCCESSION OF ANIMALS AND PLANTS IN GEOLOGICAL TIMES. 



Geologists hardly seem to appreciate fully the extent 

 of the intricate relations exhibited by the animals and 

 plants whose remains are found in the different successive 

 geological formations. I do not mean to say that the 

 investigations we possess respecting the zoological and 



