726 THE ORIGIN OF SPECIES 



changes induced in the body during Hfe could affect the germ 

 cells. This is, however, not true, nor would it be a sound argument 

 if it were. Many things happen every day, such as the falHng of 

 bodies to the earth, for which we have no mechanical explanation, 

 and in fact the existence of hormones suggests a means by which 

 any part of the body could influence any other part. There is 

 no evidence that the germ cells are influenced in this way, but 

 there is no reason why they should not be. 



Many attempts have been made to test the hypothesis of the 

 inheritance of acquired characters experimentally. Most have 

 been inconclusive for one or more reasons. Those which have given 

 negative results can be met with the suggestion that the work 

 was not carried through enough generations ; few have lasted 

 for more than five or six, which is negligible on the evolutionary 

 time-scale. Those which have given positive results have either 

 been susceptible of an alternative explanation in terms of 

 selection, or have been repeated without success. In recent 

 years, with the recognition by the geneticists of the importance 

 of the cytoplasm in inheritance, new possibiHties have emerged. 

 It is generally agreed that induced characters can be inherited 

 for several generations in the Protozoa, and there is at least the 

 possibility that the same is true of the Metazoa ; this is, however, 

 rather different from the directed production of mutations by 

 the action of the environment. 



The present theory of evolution may now be summarised. It 



accounts very satisfactorily, in terms of Mendehan inheritance, 



mutation, isolation and selection, aided perhaps by drift, for 



evolution within the genus, and brings together reasonably 



well a wide array of facts of taxonomy, ecology, distribution 



and behaviour. Its extension to the family and larger systematic 



units is progressively a matter of more and more extrapolation. 



In adaptive radiation at the ordinal level, as for example of 



mammals (Chap. 25), we can see that the same processes 



may have been at work to produce the ancestors of each order 



as have produced the species within the order. Of the evolution 



of the phyla we know nothing, and can only assume, if we choose, 



that they have been produced in the same way. At every level 



above the lowest we need to explain the origin of new genes, 



and this we cannot do. The theory of evolution, though an 



admirable working hypothesis, still leaves the most important 



things unexplained. 



