WE is MANN'S ESSAYS UPON HEREDITY. n 



an important part in Lamarck's theory of descent ; 

 indeed Lamarck would have explained the struc- 

 ture of organisms mainly through the inheritance 

 by one generation of qualities or characters 

 acquired by previous generations. The long neck 

 of the giraffe was due to constant stretching after 

 the leaves of trees, the web between the toes of a 

 water-bird's foot to the extension of the toes in an 

 attempt to oppose as large a surface as possible to 

 the water in swimming. In both cases the charac- 

 teristics of the species were supposed to be acquired 

 by minute additions from generation to generation. 



According to the theory of natural selection the 

 same facts would be differently explained ; but 

 Darwin did not altogether abandon Lamarckianism, 

 or see that all the facts could be accounted for by 

 natural selection, and that therefore the inheritance 

 of acquired characters was an unnecessary hypo- 

 thesis. Instead of this, he addressed himself to 

 the question how the use or disuse of an organ in 

 one generation could be transmitted to the next. 



The answer was the cumbrous theory of pan- 

 genesis, which Darwin himself recognized as only a 

 provisional hypothesis. It was supposed that exces- 

 sively small particles known as " gemmules " are 

 constantly given off from the cells of the body and 

 collected in the reproductive cells, so that any 

 change in the organism during its life is as it were 

 " registered for transmission " to the offspring. 



