286 EVOLUTION, OLD AND NEW. 



modification or that of " natural selection"?} be true, 

 such, have lived upon the earth." * 



To return, however, to Lamarck. 



"Though Nature," he continues, "in the course of 

 long time has evolved all animals and plants in a true 

 scale of progression, the steps of this scale can be per- 

 ceived only in the principal groups of living forms ; it 

 cannot be perceived in species nor even in genera. The 

 reason of this lies in the extreme diversity of the sur- 

 roundings in which each different race of animals and 

 plants has existed. These surroundings have often been 

 out of harmony with the growing organization of the 

 plants and animals themselves ; this has led to anoma- 

 lies, and, as it were, digressions, which the mere de- 

 velopment of organization by itself could not have occa- 

 sioned." t Ofj in other words, to that divergency of 

 type which is so well insisted on by Mr. Charles 

 Darwin. 



" It is only therefore the principal groups of animal 

 and vegetable life which can be arranged in a vertical 

 line of descent ; species and eyen genera cannot always 

 be so — for these contain beings whose organization has 

 been dependent on the possession of such and such a 

 special system of essential organs. 



" Each great and separate group has its own system 

 of essential organs, and it is these systems which can 

 be seen to descend, within the limits of the group, from 

 their most complex to their simplest form. But each 

 organ, considered individually, does not descend by 

 equally regular gradation ; the gradations are less and 

 • ' Origin of Species,' pp. 265, 266. t ' PhJl. Zool.,' torn. i. p. 121. 



