XI.] SPJiOIFIC GENESIS. 239 



osauria, Cliclonia, and Atioura), the absence of links is 

 very important and significant. For if every species, with- 

 ont exception, has arisen by minute modifications, it seems 

 incredible that a small percentage of such transitional forms 

 sliould not have been preserved. Tliis, of course, is espe- 

 cially the case as regards the marine Ichthyosanria and Ple- 

 siosauria, of which such numbers of remains have been dis- 

 covered. 



Sir William Thomson's great authority has been seen to 

 oppose itself to " Natural Selection," by limiting, on astro- 

 nomical and physical grounds, the duration of life on this 

 planet to about one hundred million years. Tliis period, it 

 has been contended, is not nearly enough, on the one hand, 

 for the evolution of all organic forms by the exclusive action 

 of mere minute, fortuitous variations ; on the other hand, 

 for the deposition of all the strata which must have been 

 deposited, if minute fortuitous variation was the manner 

 of successive specific manifestation. 



Again, the geographical distribution of existing animals 

 has been seen to present difilculties which, though not 

 tlicmselves insurmountable, yet have a certain weight when 

 taken in conjunction with all tlie other objections. 



The facts of homology, serial, bilateral, and vertical, have 

 also been passed in review. Such facts, it has been con- 

 tended, are not explicable without admitting the action of 

 what may most conveniently be spoken of as an internal 

 power, the existence of which is supported by facts not only 

 of comparative anatomy but of teratology and pathology 

 also. " Natural Selection " also has been shown to be im- 

 potent to explain these phenomena, while the existence of 

 such an internal power of homologous evolution diminishes 

 the ajy^'iori improbability of an analogous law of specific 

 origination. 



All these various considerations have been supplemented 

 by an endeavor to show the utter inadequacy of Mr. Dar- 



