THE THEORY OF N A CELL 253 



sides the external conditions regulating the varia- 

 tions of organisms. 



The Theory of Nageli. 



In regard to these internal laws governing the 

 origin of variation, science is profoundly ignorant. 

 Since it is not yet possible to say why a child is like 

 its parents, it is certainly difficult to understand 

 why it should be different. Most theories are there- 

 fore little more than suggestions thrown out as 

 perhaps indicating a truth, but as requiring more 

 evidence before they can be accepted. 



The first attempt to find an internal force, which 

 we notice, is that of the German botanist Nageli, 

 which is really little more than a statement that 

 such a force exists. He assumes the existence in 

 the organism of an internal tendency toward pro- 

 gression and perfect development, and believes that 

 in accordance with this tendency organisms are con- 

 stantly varying in such a manner as to rise in the 

 scale of nature. Here is postulated an internal 

 force, which, if it exist, is certainly able to explain 

 all the facts. The only questions are whether there 

 is any evidence for the existence of such a force, and 

 what can be our understanding of it if it does exist. 

 Nageli advances, in favor of his view, various series 

 of facts already considered, which indicate some in- 

 ternal force. He thinks that the fact of a continual 

 progression from the lower to the higher in the mod- 

 ification of species is evidence that some definite 

 tendency to advance exists. Natural selection only 

 seems to make animals better fitted to struggle with 



