GENESIS OF GENERA. 439 



tions of the several organs, their dependence on each other, and the 

 functions of the sum of the organs, we are compelled to recognise 

 variation in the work performed by the whole of the organs, as the 

 originating cause for that structural variation by which species are 

 distinguished from each other. As an organ ceases to perform work, 

 owing to the work being 110 longer necessary, or performed by some 

 other part of the body, so there comes to be not merely change in the 

 organs concerned, but in all other organs which are related to them, 

 and often in the characters and habits of the organism as a whole. 

 Just as in the individual, a certain degree of growth takes place with 

 increased work, and a corresponding atrophy follows diminished work, 

 so in wild active races bones are found to develop strong ridges and 

 crests, and muscles acquire long ligaments, which are unknown in 

 domesticated tribes ; 1 and therefore nothing is needed to produce an 

 infinity of variation in a type but ever-progressing atrophy and hyper- 

 trophy of its constituent structures, consequent on changed nutrition 

 of the structures, which has resulted from the work they have had to 

 perform. Side by side with variation in internal vital organisation, 

 variation of another kind has gone on simultaneously, which has been 

 due to the manner in which the changing conditions of existence have 

 modified the purely muscular and skeletal elements of an animal. 

 The directions of simultaneous variation may thus be numerous. If 

 the variation progresses too rapidly for the organism to become adapted 

 to the change, the modification is designated disease ; but such varia- 

 tion, though leading to extinction of races, probably has little to do 

 directly with the origin of species. The direction of physiological 

 variation is always towards increased complexity of structure, but the 

 direction of variation under external influences is often towards in- 

 creased simplicity of structure. 2 



Genus and Species. Whatever other classification may be 

 adopted, every fossil, like every living plant and animal, must be 

 referred to its genus and species, and we need to have clear ideas of 

 the nature of the facts indicated by these terms. A genus is an 

 organic variation from a group of structures such as is comprised 

 under the natural history term Ordinal group, and is thrown off with 

 distinctive characters which perpetuate the Order, in a direction which 

 its structure and the animal's habits determine to be the direction of 

 chief variation. Thus, among the bivalve shells termed Lamelli- 

 branchiata, the key to many generic modifications is found in the 

 mode in which the development and arrangement of the digestive 

 organs govern the position of the adductor muscles. In those types 

 with a single central muscle, like the oyster, scallop, lima, the digestive 

 canal is comparatively short and simple in its folds, and the axis of 

 growth of the shell is at right angles to the axis of the digestive 

 canal. But as the canal becomes more complicated, its change of posi- 

 tion results in changed position of the adductor, and the development 

 of a second adductor muscle, when the digestive tube becomes forced 



1 Darwin : " Variation of Plants and Animals under Domestication." 



2 " Origin of the Vertebrate Skeleton," Annals and Mag. Nat. Hist., November 

 1866, and April and July 1872. 



