DOCTRINE OF EVOLUTION AND RELATED IDEAS 495 



make evolution follow the demand of the environment inde- 

 pendently of the range of the variations. 



490. Variability and Variation. — All organisms vary. This 

 furnishes the materials of evolution. Variation suggests two or 

 three things: (i) An animal may change from day to day as the 

 result either of its own activities or of the action of the environ- 

 ment upon it; (2) it may differ, at any stage or condition of its 

 life, from what its parents were at the corresponding stage ; and 

 (3) it may differ also from its brothers and sisters even at the 

 same stage of life and under similar conditions. Some of these 

 changes are evidently caused by conditions outside the organ- 

 ism ; others by internal conditions. These changes make evolu- 

 tion possible. We do not believe that all these classes of changes 

 enter equally into evolution. One of the most important ques- 

 tions of the modern biologist is this : "What produces the varia- 

 tion actually found in individuals of a species, and to what extent 

 are these variations due to internal or to external causes?" 



When we actually study variations in nature we find them 

 of two principal kinds. First, and more commonly, we find what 

 are called fluctuations or continuous variations. They are so 

 called because they tend to cluster about a mean, with no 

 sharp break in the series from the lowest to the highest. We 

 should find this illustrated if we were to take at random a thou- 

 sand men and arrange them on the basis of weight or height or 

 intelligence. A curve drawn to show their distribution would 

 give the greatest number near the average and the extremes 

 would be gradually less numerous. The curve, however, would 

 be a gradual one, and we would find cases illustrating all degrees 

 within the normal range. Fluctuations are illustrated by the 

 variations in §489, i, A. 



In the second class, however, while most members of a 

 species will arrange themselves as above, we may find an occa- 

 sional individual so different from all others that there are no 

 ' ' connecting links. ' ' These are known as ' ' sports "or " mutants ' ' 

 and illustrate discontinuous variation. Experiment shows that 

 such mutations are much less frequent, but much more liable to 

 breed true, than the fluctuations. In the case of fluctuations 

 even the most extreme forms produce offspring whose average 



