442 LONGEVITY OF SPECIES. chap. xxii. 



numei'ical proportion of recent species in the newer pliocene 

 formations as compared to the older pliocene, and of them 

 again as contrasted with the raiocene; for it appeared invari- 

 ably that a greater number of the acephala or lamelli-branchi- 

 ate 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 organization 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 ani- 

 mal existence, being akin to the sponges, are extremely per- 

 sistent 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 brachiopodous 

 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 specialization of organ.* 



As one of the consequences of this law, ho 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 dichlamydeai better than the achlamydese. 



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



