488 ZOOLOGY 



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 chiefly, 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 

 if we were to take at random a thousand men and arrange them 

 on the basis of weight or height or intelligence. A curve drawn 

 to show their distribution would show 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. 



In the second case, 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 

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

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

 even the infrequent, extreme forms produce offspring whose 

 average tends to be not about the extreme parent, but rather 

 about the mean of the group. The offspring of the mutants, on 

 the other hand, have their mean about the parent, and are quite 

 independent of general mean. 



491. Heredity, the Organic Continuity between Genera- 

 tions. We have already seen (131) that the germ cells, ovum 

 and sperm, are the organic bridge which connects the body of one 

 generation with that of the next. Just as new qualities cannot 



