LIMITS OF NATURAL SELECTION. 7 



Professor Weismann sets forth. It seems that 

 among the Protozoa the single cells were changed 

 by the influences of different environment, and 

 when each cell divided itself into two or more parts, 

 the changes were naturally propagated in the dif- 

 ferent parts. Thus arose a diversity among the 

 Protozoa. When some of the Protozoa became 

 Metazoa, this diversity continued and expressed 

 itself as differences in the germ-plasms of the higher 

 group. From this point onward variation depended 

 alone on the mixture of two different germ-plasms, 

 because the germ-cells were hidden from the influ- 

 ence of the environment, and only the body-cells 

 were exposed to it. 



This theory of heredity increases the difficulty 

 in explaining evolution, as it necessitates an expla- 

 nation by natural selection alone : in addition, the 

 theory seems to have its own weak points. In the 

 first place, our knowledge of the laws of cross- 

 breeding makes it seem certain that when other 

 conditions remain unchanged, the general result of 

 crossing two dissimilar animals is an animal inter- 

 mediate in form between the two parents. If this 

 be a general law, then the original differences of 

 the Metazoa would tend to disappear, for each 

 generation would contain only forms intermediate 

 to the forms of the preceding generation. Thus 



