272 THE VARIATION OF ANIMALS IN NATURE 



on variation in the Mollusca that the form and colour of the 

 molluscan shell are very susceptible to plastic modification by 

 various environmental factors, the effects of which seem on 

 good evidence to be non-heritable. 



For many years naturalists have been familiar with the 

 resemblances between various animal structures and certain 

 inorganic phenomena — e.g. between ocellated spots and 

 Liesegang's rings. A similar parallelism has been detected 

 between the arrangement of skeletal structures and the stresses 

 set up in an animal viewed merely as a piece of engineering. 

 The subject as a whole has been dealt with at some length by 

 D'Arcy Thompson (191 7), while the special data relating to a 

 limited group, the Muscoid flies, have been ably presented by 

 W. R. Thompson (1929). The part of the first-named author's 

 argument which concerns our present discussion is his treat- 

 ment of the relation of such mechanical adjustments to the 

 problem of adaptation. D'Arcy Thompson argues that, as many 

 of the structures found in animals obey well-known laws of 

 mechanics, physics and chemistry and may be closely imitated 

 in laboratory experiments, it is unnecessary to attempt to 

 explain the adaptation of such structures as due to Natural 

 Selection. A striking example is seen in the ocellated pattern 

 on the feathers of the male Argus Pheasant, which Darwin 

 (1901) regarded as due to the selection by the female of the 

 males which pleased her best, but which D'Arcy Thompson 

 would regard as closely comparable to the Liesegang's rings 

 (formed by electrolytes crystallising out from colloid solutions) 

 and therefore as largely outside the sphere of adaptation. On 

 the Darwinian view the ocelli would be regarded as the result 

 of a long process of almost imperceptible change, each stage 

 having a slight advantage over its predecessor. On the other 

 view, while selection might have determined the persistence 

 of the ocellus-producing mechanism in the male sex only, the 

 ocelli themselves could scarcely be said to have undergone 

 evolutionary development at all. 



The structure of the bones of vertebrates provides a some- 

 what different example employed by D'Arcy Thompson (I.e. 

 chapter xvi) to illustrate the close parallelism between 

 animate and inanimate organisation. It is well known, for 

 instance, that the trabecule which fill up the greater part of 

 the end of the cavity in the long bones of the legs are arranged 



