How Plants Perpetuate Their Kinds 301 



could hardly be imagined than that which occurs through the 

 commingling of two different cells. At all events fertilization is 

 always followed, especially in animals, by that display of vigor 

 and activity which we call youth or juvenescence, whereby the 

 racial vigor is periodically renewed in each generation. Indeed, 

 so prominent and advantageous is this rejuvenescence that some 

 biologists have thought to find therein the chief utility of sexual 

 reproduction. Perhaps it does indeed play some part, for sexual 

 reproduction, like many other physiological processes, is probably 

 not the expression of a single factor, but the resultant of the co- 

 operation of several. 



Replacement of the individuals which must die is no doubt the 

 first meaning of reproduction, but therewith is often associated 

 the idea of multiplication in number. Multiplication, however, 

 is more seeming than real, as shown by this fact, that in general 

 any kind of animal or plant, no matter how numerous its off- 

 spring, does not alter its numbers appreciably from one year to 

 another. Thus, in general, there are no more Mushrooms, Dan- 

 delions, or Robins in a given county this year than last, and the 

 numbers of each kind remain for decades substantially stationary. 

 Even the occasional exceptions caused by the introduction of new 

 weeds or animal pests, or by the expansion of man himself, is no 

 real exception, for after a time these also attain a condition of 

 numerical stability. Hence, the offspring formed by animals and 

 plants do not in general increase their numbers, but simply make 

 up for losses. In reproduction, therefore, multiplication is sub- 

 ordinate to continuance of the kind. 



Reproduction, as we have seen, is essentially Nature's method 

 of continuing the kinds of plants or of animals as the individuals 

 perish. This being true it follows that if the individuals were 

 immortal, there would be no need for reproduction, after once 

 the world was fully populated. This view receives confirmation 

 from the balance which exists between the vegetative prosperity 

 of the individual and its reproduction, anything favoring the one 



