TRACHEATES 223 



untouched. But in these cases the germ is not modified 

 in a particular direction, but is diseased, like the whole 

 body. This is what we have in cases of syphilis or 

 alcoholic poisoning. The infection of the germ is 

 brought about either by minute organisms that pene- 

 trate to it, or by alcohol, which naturally reaches the 

 germ as it courses through the whole body. 



The Lamarckian principle cannot, therefore, help us 

 in explaining certain characteristics of animals, because 

 it does not explain many of them ; in the second place, 

 it cannot be proved ; and in the third place, It is not an 

 explanation at all. 



Natural selection rests on two causes, both of which 

 have been demonstrated — on the variations of animals 

 and their inheritance, and on over-production and the 

 destruction of a certain percentage which this entails. 

 But of the two postulates of the Lamarckian principle 

 only one has been established, and this only in certain 

 cases. This is the statement that an organ is strengthened 

 by use and enfeebled by disuse. The second postulate 

 — that these changes affect the germ in precisely the 

 same sense — is a hypothesis that has not only never 

 been proved, but itself requires explanation. 



We must certainly admit that the ova^ — and also the 

 sperm — that mature within an animal are infinitely com- 

 plicated structures ; and that in each ovum there is an 

 enormous number of minute particles, each of which 

 forms a certain organ when an animal develops from the 

 ovum. But these particles are not miniatures of the 

 subsequent organ ; they have a totally different form. 



Hence the stimulus that modifies any organ of the 



