NATURAL SELECTION 135 



Measurements and observations of other animals and of plants 

 brought out similar results. If, therefore, a slight increase in 

 any particular dimension is of advantage, there is an ample 

 supply of individuals in which this favourable variation is 

 found, and if it can be proved that their slight superiority 

 makes the difference between survival and non-survival, then, 

 it is maintained, all is plain sailing, and evolution pursues its 

 way on strictly Darwinian lines. 1 Our first thoughts when we 

 reflect upon this theory may well be that there are some varia- 

 tions that are not of so simple a nature. 



It cannot be denied that the variations in a particular dimen- 

 sion can be explained as deviations from a mean, and variations 

 in the quality of tissue might possibly be described as deviations 

 from a chemical mean. But there are some examples which 

 hardly seem to be covered by Professor Weldon's definition. 

 To take an instance from Mr Bateson's Materials for the Study of 

 Variation (p. 201), there is in the Oxford Museum an orang- 

 outang which has all its teeth normal except the second premolar 

 on each side of the upper jaw. On each side there is a gap 

 where the tooth should be. The missing tooth on the right 

 side stands immediately in front of the canine, and has exactly 

 the form of the second premolar. Something has pushed it out of 

 position. The plan of the building has somehow got disarranged. 

 But what is the mean from which there has been a deviation ? 



The following examples are no less difficult to explain on this 

 principle. 



Bilateral symmetry is the rule among all the higher classes of 

 animals. But there are some notable departures from it. In 

 snakes, of the two lung-pouches, one is reduced to the very 

 smallest dimensions ; in birds the lobes of the liver are unequal 

 and one of the two ovaries has disappeared, the left aortic arch is 

 missing ; in mammals the left arch survives ; the left tusk of the 

 narwhal is extraordinarily developed, the right is small; domestic 

 rabbits sometimes have but one ear. 2 In these cases also we 



1 See Nature, Sept. 22, 1898, where Professor Weldon's paper on the subject is 

 reported. 



2 Darwin : Animals and Plants under Domestication, vol. ii. p. 12. 



