EVOLUTION AND HEREDITY 351 



from parent to offspring. Though all the problems have 

 not been solved, the chief kinds of variations have been 

 determined and some fundamental laws of heredity have 

 been worked out. 



Fluctuating Variations and Mutations. Some character- 

 istics vary according to the laws of chance on either side of 

 an average condition. For example, the height of man is 

 about five feet eight inches and it has been determined by a 

 large number of measurements that the farther men depart 

 in stature from this average, the fewer they are in numbers. 

 There are many men whose height is five feet seven inches or 

 five feet ten, but those that measure seven or five feet are 

 rare. Such departures from the average of a species are 

 called indeterminate, fluctuating, or " chance" variations. 

 Sharply separated from these are those variations which 

 show no gradations, but are clearly differentiated in each 

 individual. The short-legged Ancon sheep appeared sud- 

 denly in stock which had previously possessed legs of normal 

 type. When Ancons are bred with other sheep the off- 

 spring have long or short legs; there are no intermediate 

 conditions. Such new variations which hold their own in 

 heredity are commonly called mutations. 



Orthogenesis. Where variation proceeds in a particular 

 direction through succeding generations it is said to be ortho- 

 genetic. Eimer was the first to point out that many varia- 

 tions do not follow the laws of chance but run in particular 

 grooves. This discovery has been of great value in the 

 understanding of certain points which were not readily 

 explained by natural selection, such as cases of mimicry 

 of one species by another and the very elaborate color 

 patterns developed by some species. It makes the broad 

 differences between groups of animals more easy to under- 

 stand: e.g., why the insects always have three pairs of 

 legs; the spiders, four; and the vertebrates, two. 



Somatic and Germinal Variations. Weismann has made 

 a distinction between variations which occur in the bodies 

 of metazoans and those which influence their germ cells. 



