THE ORIGIN AND DEVELOPMENT OF ORGANISMS 327 



Variation. The offspring of any animal is never exactly like 

 either of its parents. Sometimes it is a compromise between 

 them; sometimes, for certain reasons that we do not under- 

 stand, it is quite different from either. The reasons why any 

 peculiarity may reappear in successive generations, are probably 

 partly due to processes connected with the reproductive func- 

 tions, but they are also partly due to the effect of the environ- 

 ment in which the animal lives, upon the structure, the nature, 

 and the life of the organism. Whatever be their cause, the 

 points in which animals and plants differ from each other, or 

 from their ancestral types, are known under the general name 

 of variations. 



The life of an individual which is produced by sexual repro- 

 duction may be said to begin at the moment when the sperm 

 fuses with the egg, as shown in Figure 121. Previous to this, 

 there were only the sex cells produced by two parents; but 

 from this point there is a new individual resulting from the 

 union. Variations which appear in an animal or a plant may 

 be caused by influences acting either before or after the union 

 of the sex cells. If the variation is caused by influences acting 

 before this union, we speak of it as a congenital variation (Lat. 

 con = together + genitus = produced) . If, however, the varia- 

 tion is developed in the animal after the fusion of the sex cells, 

 and thus produced by influences acting directly on the new 

 individual, we speak of it as an acquired variation. Although 

 this distinction between acquired and congenital variations may 

 be merely a matter of a short time, nevertheless the facts 

 show that there is a very great distinction between character- 

 istics produced before and after this period. Variations which 

 are produced by influences acting before the fusion of the 

 sex cells (congenital variations) are practically certain to be 

 handed to the subsequent generations by heredity. Variations 

 which arise subsequently, and affect the new individual only 

 (acquired variations), are practically certain not to be handed 

 on to the following generations by heredity. 



