34 THE PRINCIPLES OF HEREDITY 



development is maintained only as a response to this stimu- 

 lation (i. e. use, exercise) ; for example, when not used, the 

 muscles and their co-ordinated structures atrophy and tend to 

 disappear, as in the case of a paralyzed limb. It may be 

 added that it is probable that even the infantile standard of 

 development is to some extent acquired under the stimulus 

 of foetal movements in utero. 1 Adult man consists therefore 

 of a huge superstructure of use-acquirements, built on a 

 comparatively slender foundation of inborn traits. 



56. In upholding the doctrine of the transmissibility of 

 acquired modifications, much stress has been laid by Mr. 

 Herbert Spencer and others on the exquisite co-ordination of 

 the multitudinous parts of the high animal organism. They 

 maintain that this co-ordination affords decisive proof of the 

 Lamarckian doctrine, the line of argument being as follows : 

 It is not probable that the many structures of a high animal 

 can ever have varied favourably together (as compared to 

 the parent) in any individual animal. It is unbelievable 

 that they can all have varied favourably together, generation 

 after generation, in a line of individuals. A chain is only as 

 strong as its weakest link. A favourable variation, say a 

 large horn in the elk, if unaccompanied by corresponding 

 variations in all the thousand parts (in head, neck, trunk, 

 limbs) co-ordinated with it, would be useless and even 

 burdensome. In other words, if a single structure (muscle, 

 bone, ligament, blood-vessel) of all those associated with the 

 large horn, failed to bear the strain, the larger horn would 

 not favour survival ; but, on the contrary, would be a cause 

 of elimination. Therefore, say these thinkers, the evolution 

 of high multicellular animals cannot be attributed to the 

 accumulation during generations of " spontaneous " variations 

 only, but must, in part at least, be attributed to the inherit- 

 ance and accumulation, during generations, of the effects of 

 use and disuse. 



57. But modifications, acquired as a result of use and 

 disuse, are clearly never transmitted. Thus, an infant's limb 

 never attains to the adult standard except in response to 

 stimulation similar to that which developed the parent's 

 limb. The same is true of all the other structures which in 

 the parent underwent development as a result of use. These, 

 like the limbs, do not develop in the infant, except as a 

 result of similar causes. Yet all such characters must have 



