70 THE LAMARCKIAN DOCTRINE 



relieved of unnecessary burdens. The capacity to develop under 

 the stimulus of use, therefore, renders the individual more adapt- 

 able than he would otherwise be. In fact, he is doubly adapted 

 to the environment : first by the evolution of his race, which fits 

 him to the environment common to all the members of his 

 species, and second by peculiarities in his own development, which 

 arise under the stimulus of use and injury and fit him to his own 

 special surroundings. 1 



in. Yet another quality of special usefulness is conferred by 

 the capacity to develop under the strain of use. The only organ- 

 isms which possess this power to any considerable extent are the 

 more complex animals. Thus, a man is compounded of thousands 

 of structures, all of which must be closely co-adapted, or the 

 individual as a whole lacks efficiency. The greater the complexity, 

 the more necessary but difficult is nature's task of maintaining 

 co-adaptation. By the evolution of the power of growing under 

 the strain of use the difficulty is in great measure surmounted, for 

 each structure thus endowed develops in proportion to the strain 

 placed on it. The size and strength of the human heart, for 

 example, is, in general terms, always in proportion to the needs ot 

 the individual, be he healthy or diseased, big or small especially 

 in early life, when the power of developing under the strain is 

 greatest. On the other hand, the magnitude of such structures as 

 the teeth, the ears, the nose, the hair, and the reproductive organs, j 

 all of which are wholly 'inborn,' is often comparatively dispro- 

 portionate as compared with associated parts. 2 



112. It follows that if a character which developed in the j 



I 



1 See chapter xxi. 



2 Much has been written on the ' correlation ' of parts and qualities. The word | 

 has been used to describe at least three distinct classes of phenomena. First, there 

 is the infrequently observed but much discussed tendency of parts, which have 

 apparently no close functional connection, to vary together in a non-adaptive j 

 way in a way that has not resulted directly from evolution, but is seemingly a 

 kind of by-product of it. Thus " white cats which have blue eyes are almost 

 always deaf" (Darwin, Animals and Plants, vol. ii. p. 322) ; "multiplicity of 

 horns in sheep is generally accompanied by great length and coarseness of the j 

 fleece" (Youatt on Sheep, pp. 142-69, quoted by Darwin). "White terriers 

 suffer most from distemper, white chickens from a parasitic worm in their trachea, . 

 white pigs from scorching in the sun, and white cattle from flies " (Vernon, ! 

 Variations in Plants and Animals, p. 85). A normal individual (e.g. man) is ait 

 bundle of adaptations. He consists of a thousand or ten thousand parts, each :| 

 of which is co-adapted, not merely to a single other, but to many others, and must ij 

 have been so co-adapted during all the shifts and changes of evolution. It is 

 difficult to understand how this perpetual co-adaptation in all directions can have j 

 been secured unless each part has been independently variable. Only so can 

 the species have been sufficiently ductile to the action of Natural Selection. We j 



