140 



EVOLUTION AND ANIMAL LIFE 



"Whatever the cause may be of each slight difference between the 

 offspring and their parents — and a cause for each must exist — we have 

 reason to beUeve that it is the steady accumulation of beneficial defer- 

 ences which has given rise to all the more important modifications of 

 structure in relation to the habits of each species." 



Num. of 

 Variftfes. 



Modern investigation of variation, which includes at least two 

 phases of study that have been developed since Darwin's time, 

 namely the statistical and quantitative, and the experimental 



studv of variation, has been 

 able to add much information 

 about the mode and the 

 character of variations, and 

 has effected a sort of classifi- 

 cation of them which helps at 

 once to express and to clarify 

 and organize our knowledge 

 of variability. But it has 

 added as yet no great funda- 

 mental generalizations really 

 worthy to be called laws. 



One generalization there is, 

 perhaps, of application and 

 value far reaching enough to 

 be called law (although it ap- 

 plies to only a single category 

 of variation, but a large one), 

 and that is the law formulated 

 in 1846 (ten years before Dar- 

 win's "Origin of Species"), 

 by the Belgian anthropologist, 

 Quetelet, on the basis of the 

 examination of the height and 

 chest measurements of soldiers. As it applies only to what 

 are variously called fluctuating, individual, continuous, or Dar- 

 winian variations, we may note before stating the law the cur- 

 rent mode of classifying the variations which occur in plants 

 and animals. 



Variations may be either congenital or acquired: that ic, 

 may be such as are apparently determined in the organism £/: 

 conception, or such as are imposed on it during its development 



Cksses-.O 1 234561 89 10 



Fig. 83. — Diagram showing curves of 

 distribution of frequency of variation in 

 glands of swine. (After Davenport.) 



