1870.] 



Tlie Bate of Geological Change. 



325 



the Mesozoic 2170, and the Cainozoic 1222. "The absolute 

 number of marine species appears thus to be greatest in the 

 Paleozoic strata; but when the thickness of the deposits, which 

 represents elapsed time, is taken into account, the variety of forms 

 hi a given thickness or given period of time is very much less." 

 This conclusion he illustrates by the following Table : — 



Cainozoic strata . . 

 Mesozoic strata .. 

 Palaeozoic strata . . 



Total Species in 

 eight Classes. 



1,222 



2,170 

 2,729 



Maximum 

 Thickness. 



2,240 

 23,190 

 57,154 



Relative Number 



of Species to 



1000 feet. 



545 

 164* 

 41f 



And he again observes (p. 62), " Thus it appears certain that the 

 variety of life estimated by the marine tribes existing in a given 

 period is greater in the more recent periods.''^ 



* Should be 93. f Should be 47. 



% The editor of this Journal (Mr. James Samuelson) has asked me whether I 

 may not have overlooked the biological aspect of the case, and whether this may 

 not be the result of the laws of selection in the lower forms of life varying from 

 those in the higher forms, and the rate of change being perhaps more rapid in the 

 latter ; e. g. low molluscs or acalephs having predominated in earlier times, might go 

 on multiplying for ages without material change, their conjugation being, like that 

 of plants, regulated by the elements, whilst in the fishes and higher molluscs, sexual 

 selection and destruction of each other would be very active and would produce 

 rapid change of species. It appears to me that at least two powerful arguments 

 may be advanced against this interpretation. First, geologically considered, the 

 interpretation would be erroneous because we are contrasting the abundance and 

 variety of life characteristic of the different great periods, and it cannot be said of 

 any one great period that it is characterized, for instance, exclusively by low mol- 

 luscs, nor of any other that it is characterized exclusively by higher orders of that 

 class, as is shown in the following Table by Professor Phillips : — 





Zoo- 1 Echino- 

 phyta. derma ta. 



Crus- 

 tacea. 



Brachio- 

 poda. 



Mono- 

 myaria. 



Dimy- 

 aria. 



Gaste- 

 ropoda. 



Cepha- 

 lopoda. 



Total. 



Cainozoic . . 

 Messozoic . . 

 Palaeozoic .. 



27 

 103 

 379 



41 



245 

 225 



15 



65 



218 



8 

 165 

 632 



63 

 308 

 196 



394 

 499 

 342 



662 

 389 

 401 



12 

 396 

 336 



1222 

 2170 

 2729 



Thus the Palaeozoic rocks abound both in the highest and the lowest orders 

 of the testaceous Mollusca, viz. the Cephalopoda and the Brachiopoda, while the 

 Tertiary deposits are characterized by the intermediate groups of Gasteropoda 

 and Lamellibranchiata. The oldest fishes are not regarded by the best autho- 

 rities as of uniformly lower types than the most recent ; and with regard to the 

 power of sexual selection, the advantage is doubtless on the side of the Palaeozoic 

 and Mesozoic representatives of the modem Claspers. But the weightiest argument, 

 to my mind, is to be derived from the improbability that sexual selection in such 

 animals (for, owing to our imperfect record, we are necessarily compelled to deal 

 almost exclusively with animals of no high zoological grade) was to any great 

 extent the result of direct volition, or that sexual selection was the determining 

 cause of change of species, although it no doubt was one means to that end. 

 It appears to me far more probable that changes in species have been rapid 



