43 8 THE POPULAR SCIENCE MONTHLY. 



Their presence implies an unceasing and more or less violent struggle 

 between the tendency to an untimely death and the natural term of life. 

 The prolonged existence of such variations, especially when they affect 

 the more important organs, is scarcely possible ; progressing in acute 

 attacks, or through descent, either toward reversion or toward lineage 

 extinction. Indeed, if abnormities were in their nature as permanent 

 and compatible with the life of the blood as normalities, they would 

 cease to be such, and all distinctions between them would vanish. 

 Some deviations, however, from ordinary development, such as poly- 

 dactylism, can not be considered morbid, and are in no way incompat- 

 ible with the continuance of life. 



Organic variability is mainly, if not entirely, an effect of changes 

 in condition. The increased use or disuse of any part, a change from 

 one climate to another, and of the quantity or quality of food, have 

 long been noticed to be specially influential. The various species of 

 domesticated plants and animals are far more variable than the same 

 when wild, a difference readily accounted for by the fact that the lat- 

 ter are far less migrant ; their nutriment and the culture of parts un- 

 dergo little if any change in great lapses of time. Some variations, 

 like that of the loss of wool which our sheep undergo when ]naced 

 within the tropics, are apparently grounded upon adaptation to rela- 

 tion, others upon adaptation to use, and others are dependent on me- 

 thodical selection. 



One and all of these diverging tendencies are opposed by a single 

 conservative principle, that of reversion. The tendency of animals 

 long domesticated when they become feral to revert to the pristine 

 type might also be supposed to arise from adaptation to relation. But 

 the tendency is often seen when there has been no apparent change in 

 the conditions, as in the many and familiar instances of atavism ; and 

 more decisively, by the crossing of species. The experiment of the 

 Earl of Powis is one example of the many which evince the marked 

 tendency to reversion through the crossing of blood. He had some 

 domesticated hump-backed cattle crossed by the wild species from In- 

 dia, with the result, not of producing a medium grade of characteris- 

 tics, but of a marked reversion to the ancient.* Here, adaptation to 

 relation can not be regarded as effective ; only through the indelibility 

 of types by prolonged transmission is such a prepotency explicable. 



The principle of reversion is the touchstone of the fitness for sur- 

 vival of blood variations normal in character. If they withstand for 

 indefinite generations the tendency to be supplanted by older and more 

 deeply fixed characters, their fitness is proved, and at the same time 

 they acquire prepotency for strong maintenance against fresh devia- 

 tions. The principle is therefore eminently conservative, and not a 

 mere blind tendency of the organic processes. Unlike the coarse and 

 obtrusive struggle for existence between independent organisms, it is 

 * "Animals and Plants under Domestication," vol. ii., p. 19. 



