1887.] 1-5 [Claypole. 



V. 



Indefinite Variation. 



The instances already given have sufficiently illustrated the fact of pre- 

 judicial variation. Of beneficial variation no evolutionist entertains any 

 doubt. To dwell on it will therefore be needless. But there is a third 

 aspect of the change which must not be omitted. As we have seen, the 

 gain or loss of an organism by varying may be of any degree from that 

 which gives the variate a surpassing advantage and predominance over his 

 fellows to that which leads straight to extinction. In mathematical lan- 

 guage the range of variation is from positive infinity (+ <^) through zero 

 to negative infinity ( — oo). We must consequently admit the existence of 

 variation which confers no advantage and inflicts no disadvantage on the 

 variant — neutral variation it may be called. This neutral variation is an 

 important factor in the problem, though hitherto it has received very little 

 attention. It is capable of explaining some difficulties, of removing some 

 anomalies. Darwin has alluded to it in a single passage : "I am inclined 

 to suspect that we see in polymorphic genera variations in points of struc- 

 ture which are of no service or disservice to the species " (Origin of Spe- 

 cies, p. 46). 



Variation of the kind now under consideration may be often seen among 

 the domestic animals where the struggle for existence is less severe and 

 controlled by other laws than among the wild species. For example, six- 

 toed cats (see "Nature" for 1886 and 1887) are a not uncommon though usu- 

 ally a local variety. The peculiarity is freely transmitted. Yet no ill 

 effect seems to attend the irregularity. Indeed if, as asserted, they are 

 good mousers, it may confer a slight benefit though it detracts much from 

 a light and graceful appearance. The same variation is not uncommon 

 among mankind, is there also freely transmitted and is also equally inert 

 in result. The tailless Manx cats may also be quoted in the same connec- 

 tion,* the great range of color in the domestic animals and the manifold 

 shapes of the leaf in many of our garden vegetables which are reproduced 

 with certainty and seem to work neither good nor evil to the plant. 



Among wild species the same fact may be noted. Great difference may 

 be seen among the leaves of any species of our forest trees attended with 

 no perceptible advantage or disadvantage. In these cases we need not be 

 surprised to see the variates living side by side with their unvaried ances- 

 tors. The red maple of North America is a striking instance. This tree, 

 whose remains are found fossil in the Miocene strata, yet lives in company 

 with its more highly developed and later variates which do not occur in 

 the fossil state. 



On this principle I would explain the fact brought forward by Dr. Car- 

 penter of the existence of ancestral forms of Orbitolites alongside of later 



* In a case that recently occurred under my own observation a single kitten of a litter 

 was born without a tail. It is now nearly full grown and appears to suffer no inconve- 

 nience from the curtailment. In a similar way the Manx cats may have originated. 



