442 ' LONGEVITY OF SPECIES. CHAP. xxil. 



numerical proportion of recent species in the newer pliocene 

 formations as compared to the older pliocene, and of them 

 again as contrasted with the miocene ; for it appeared invari 

 ably that a greater number of the acephala or lamelli- 

 branchiate bivalves could be identified with living species 

 than of the gasteropods, and of these last a greater number 

 in the lower division, that of entire-mouthed univalves, than 

 in that of the siphonated. In whatever manner the changes 

 have been brought about, whether by variation and natural 

 selection, or by any other causes, the rate of change has been 

 greater where the grade of organisation is higher. 



It is only, therefore, where there is a full representation of 

 all the principal orders of mollusca, or when we compare 

 those of corresponding grade, that we can fully rely on the per 

 centage test, or on the proportion of recent to extinct species 

 as indicating the relation of two groups to the existing fauna. 



The foraminifera which exemplify the lowest stage of 

 animal existence, being akin to the sponges, are extremely 

 persistent throughout vast periods of time in form and 

 structure, as the researches of Messrs, Jones and Parker have 

 lately shown. They exceed, in that respect, even the brachio- 

 podous mollusca before mentioned, 



Dr. Hooker observes, in regard to plants of complex floral 

 structure, that they manifest their physical superiority in a 

 greater extent of variation, and in thus better securing a suc 

 cession of race, an attribute which in some senses he regards 

 as of a higher order than that indicated by mere complexity 

 or specialisation of organ.* 



As one of the consequences of this law, he says that species, 

 genera, and orders are, on the whole, best limited in plants 

 of higher grade, the dicotyledons better than the monocoty 

 ledons, and the dichlamydese better than the achlamydese. 



* Introductory Essay, &c., p. vii. 



