26 THEORY OF EVOLUTION 



the pigments that have gone into the painting 

 of his picture, and he may confuse a familiarity 

 with the different views he has enjoyed of the 

 canvas with a knowledge of how the painting 

 is being done. 



My good friend the paleontologist is in 

 greater danger than he realizes, when he leaves 

 descriptions and attempts explanation. He 

 has no way to check up his speculations and it is 

 notorious that the human mind without con- 

 trol has a bad habit of wandering. 



When the modern student of variation and 

 heredity the geneticist looks over the differ- 

 ent "continuous" series, from which certain 

 "laws" and "principles" have been deduced, he 

 is struck by two facts: that the gaps, in some 

 cases, are enormous as compared with the single 

 changes with which he is familiar, and (what is 

 more important) that they involve numerous 

 parts in many ways. The geneticist says to the 

 paleontologist, since you do not know, and 

 from the nature of your case can never know, 

 whether your differences are due to one change 

 or to a thousand, you can not with certainty 

 tell us anything about the hereditary units 



