SUMMARY 725 



chemical nature of the gene, but so far as our evidence goes such 

 changes seem to take place by chance ; that is, we can find no 

 reason why the change should take place at one meiosis rather 

 than another. We can increase the rate at which mutation occurs, 

 that is, the proportion of genes which change in a large number 

 of meioses, by such agents as X-radiation, high temperature 

 and chemicals, but in this we do nothing new ; we merely increase 

 the existing instability of the nucleus. Even if we did know of 

 a mechanism by which we could cause genes to mutate in a 

 particular direction, we should still know little about evolution 

 above the species level, for here we need an explanation of the 

 origin of new genes. We know of a group of genes which produce 

 wings, absence of wings, small wings, and various types of ab- 

 normal wings in Drosophila, but for a complete theory of 

 evolution w^e need to be able to suggest a mode of origin of these 

 genes in the primitive wingless arthropod ancestor from which 

 we assume Drosophila to be descended. 



Some of the earUer writers on evolution saw this difficulty 

 more clearly than do many of their successors. Buffon supposed 

 that the direct effect which the environment undoubtedly often 

 has on an organism, for instance the tanning of the skin in man 

 by sunlight, would be inherited to some extent by its offspring 

 even in the absence of the particular environmental stimulus 

 that produced it. Lamarck (1744-1829) thought that every animal 

 would change in accordance with its needs. In part this was a 

 semi-mystical conception, suggesting for instance that a shrew 

 by trying to fly might become a bat, but it also included the 

 supposition that the effects of use and disuse are inherited ; an 

 animal which ran fast would increase the size of its leg muscles 

 and so would leave offspring which started with slightly better 

 legs than their parent at the same age ; a cave animal which did 

 not use its eyes would have offspring with slightly smaller or 

 weaker eyes, and so on. All these theories involve the inheritance 

 of acquired characters, that is at least the partial transmission 

 to descendants of characters acquired during the lifetime of the 

 individual. Darwin thought that this was possible, but unimport- 

 ant. Later writers, including the earlier Mendehans, thought 

 that it was theoretically impossible because the cytoplasm 

 played no part in heredity and the germ-cell nuclei were isolated. 

 This view is still widely held, and it is still sometimes said that 

 it is difficult or impossible for us to conceive any means by which 



