340 BOTANY OF THE LIVING PLANT 



by rejection of the less fit, and survival of the fittest. Its recognition as 

 a feature affecting all wild life was a novel factor introduced into 

 biological Science by Darwin. 



The efficiency of Natural Selection as a means of accumulating 

 new characters depends upon the fact that the production of germs 

 is carried on at a higher rate than would suffice simply to restore 

 losses by death. This has already been shown for Flowering Plants 

 in Chapter XIX., p. 332, where extreme cases of prolific propagation 

 have been quoted. The lower organisms as a rule reproduce pro- 

 lifically also, and often with extreme rapidity. An overplus of young 

 individuals is indeed a general feature of plant-life. Since each of the 

 offspring is normally capable of reproducing as prolifically as the 

 parent, if all came to maturity the numerical increase would be in 

 a geometrical ratio. But all of the individuals produced do not and 

 cannot come to maturity, so as again to propagate their species. 

 There could never be room or opportunity for nutrition of them all 

 upon the earth's surface. Their number leads to a competitive 

 struggle for existence. In ordinary vegetation the number of estab- 

 lished plants remains approximately constant in any given area as 

 the result of that struggle. If this be so, the average mortality of 

 the offspring of an annual would involve all of the germs of each 

 season but one : and in the case of perennials only a fraction of that 

 number of survivors would come to functional maturity. It is upon 

 the enormous overplus of individuals produced that Natural Selection 

 works. The most likely to survive are the strongest, or those which 

 present some favourable variation from type. Those that are weaker 

 or less suited to their environment are the more likely to succumb. 

 The result of Natural Selection will therefore be the Survival of the 

 Fittest together with the perpetuation of favourable variations from type. 



A special Mechanism of Heredity is shown to exist by the facts of 

 sexual reproduction, as seen in the higher types both of Plants and 

 of Animals. Such reproduction is referable to the sexual cells alone. 

 In Flowering Plants it has already been seen that the new individual, 

 with all its characteristics of structure and function, is derived from 

 the ovum fertilised by the male gamete. These cells must then be 

 the bearers of all those characteristics which are transmitted from 

 the parent to the offspring. For the maturing of the sexual cells, 

 and the part which their nuclei are believed to take in the segregation, 

 transmission, and distribution of hereditary features, reference must 

 be made to Chapter XXXV., where the details of the chromosome- 

 cycle are given for the Higher Plants, together with a brief account 



