41 



two are alike, new combinations of the germ-plasm are continually aris- 

 ing, and these express themselves in still other individuals which are 

 different from any that have ever lived. Amid all these variations, which 

 indeed will affect every organ, are some which are hurtful to the organ- 

 ism, and others which are advantageous. Such variations will come 

 under the influence of natural selection, the individuals possessing hurt- 

 ful variations being destroj^ed, those with advantageous variations being 

 preserved and made the means of transmitting on to future generations 

 the improvement. (Organic evolution, then, according to Weismann, de- 

 pends on two factors, variation brought about by sexual mixture, and 

 natural selection. Indeed, according to him, the production of variations 

 that may be inherited constitutes the whole significance of sex ; it is sim- 

 ply a device of nature for the origination of variations through which 

 natural selection may effect improvement. As a corollary from this prop- 

 osition Weismann deduces the conclusion that any organisms which do 

 not reproduce sexually, such as certain i)arthenogenetic insects and cru.s- 

 taeeans, cannot undergo variation; and should their environment change 

 to any considerable degree they must perish. However, since the ])ubli- 

 cation of his lectures, Weismann has been compelled t<> recede from this 

 position. 



But if it be true that external influences have had nothing directly to 

 do in bringing about inheritable changes in organisms, and if the species 

 of one age have descended from more ancient species, how did the hered- 

 itary individual differences arise in the beginning? With most other 

 evolutionists he believes that the Metazoa have been derived from the 

 Protozoa. In the Protozoa, there is no reproduction by means of eggs. 

 The animal is at once parent and egg. When reproduction occurs, it is 

 usually accomplished by the division of the animal into two portions of 

 equal size and similar form, so that it is impossible to say that either is 

 parent or offspring. Each part reproduces in a similar way ; and since 

 there appears to be no reason why, in case the environment remains 

 favorable, any of the products of division should ever die, AVeismann re- 

 gards them all as having potential immortality. 



It must be remembered now that AVeismann admits that external forces 

 and conditions, as well as the use anil disuse of organs, may affect pro- 

 foundly the organization of even the higher animals, although he denies 

 that any of the direct effects will be passed on the next generation. In 

 like manner the Protozoan is influenced by external conditions and would 



