148 HEREDITY AND SELECTION IN SOCIOLOGY 



In the phenomenon of the origin of species, more, perhaps, than 

 in any other, we see the action of selection.^ 



The life-history of species is sometimes compared to that of 

 organisms. Species, it has been said, imdergo, like organisms, 



^ The case of the luminous organs of deep-sea fishes is particularly, 

 instructive. In some instances we find glands which secrete a luminous 

 substance ; in others we have before us a complicated organ, resembling a 

 lantern, from which a ray of Ught is projected Kke a flash from a Hght- 

 house. This apparatus possesses a highly developed innervation, and its 

 origin can certainly not be due to mutation — that is to say, to the acci- 

 dental sudden emergence of discontinuous variations in a number of 

 individuals without relation to the conditions of external Ufe. As little 

 can we attribute its origin to climatic influence, for not every species of 

 flsh possesses this luminous apparatus, but only some deep-sea species ; 

 and yet the climatic conditions must act in a manner which is not very 

 dissimilar on all the inhabitants of the different zones of the ocean. At aU 

 events, these conditions are not sufficiently heterogeneous to account for 

 the heterogeneity of luminous organs among different species. The 

 transmission of the effects of use and disuse is, as we have seen, highly 

 improbable, and — at any rate, in the present instance — ^it can find no 

 appUoation. These comphcated " lanterns " are found in very different 

 species of deep-sea fish, and their utihty as an aid in catching prey is 

 probable ; for Chun has pointed out that they serve to attract the smaller 

 fish, just as the electric lamp attracts insects. But not aH the different 

 species have the luminous organs on the head or jaws. Some have them 

 on the flank, others on the ventral surface ; and Weismann has pointed 

 out how very probable it is that these organs on the flank or ventral sur- 

 face serve to illuminate the floor of the ocean, and to render prey visible 

 in the darkness. The theory of Delage and De Vries is incapable of 

 showing why these luminous organs should have arisen in the deep-sea 

 fishes and not in those species whose habitat is nearer the surface ; whereas 

 their origin becomes easily expKcable if we see in such organs a case of 

 adaptation to peculiar circumstances, to the obscurity of the ocean depths. 

 Those individuals which exhibit a variation in the direction of luminosity 

 must have been selected, and thus the evolution of the peculiarity would 

 advance until it was completely adapted. 



The deep-sea fishes afford another instance of adaptation in their eyes, 

 which are often unusually large. Every organ is equally the result of 

 adaptation, as is also the entire nervous system. The whale among 

 marine animals is one of the most striking examples of adaptation. It 

 belongs to the class of mammals, but its body has wholly changed its 

 shape, having become similar to that of a fish. The hind-legs have dis- 

 appeared entirely, being represented only by vestiges within the muscular 

 system ; the fore -legs have been transformed into flippers, but the usual 

 skeletal parts are all present. The hairy covering of the ancestors has now 



