[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 501 



vival of the fittest," they have been accepted and used freely with a 

 variety of imphcations by some biologists, adversely criticized or re- 

 jected by others. Those who object to the terms regard them as slogans 

 that are used without an adequate analysis of the conditions from which 

 they are inferred or to which they are applied. Such scientists think the 

 terms "random survival" and "random elimination" are more in accord 

 with the established facts. 



For example, in a natural climax forest undisturbed by man, the area 

 is fully occupied by all the plants that can grow there. Countless num- 

 bers of seedlings may be found in such forests, but the survival and 

 growth of any one of them to maturity depend upon the death of some 

 established plant. Even these innumerable seedlings are but a small per- 

 centage of the seeds that began to develop, because many of them are 

 eaten by animals and others are destroved by fungi and bacteria, or by 

 other means. 



Since the embryo in the seed develops from a fertilized egg, there are 

 hazards that precede the formation of embrvos. Out of the cloud of 

 pollen grains containing sperms, or the generative nucleus from which 

 sperms are formed, perhaps not more than one potential sperm in a 

 trillion, or several trillions, ever unites with an egg. If any one of the rare 

 speiTns that unites with an egg, or an\' one of the relatively few seeds 

 that germinate, or of the seedlings that grow to maturitv happens to be 

 the "fittest" of the lot, would it be a case of "natural selection" or of 

 random survival? 



Before the advent of man, biological conditions in the natural vegeta- 

 tion throughout the world were comparable to those in the forest de- 

 scribed above. Erosion, flooding, fires started by lightning, volcanic erup- 

 tions, aridity, and glaciation occasionallv destroved some of the vegeta- 

 tion locally and increased the areas in which seedlings might develop to 

 maturitv. 



Furthermore, much that has survived is of no particular value to any 

 plant. You have learned to distinguish several different kinds of trees by 

 distinctive hereditary characters, such as leaf form, leaf arrangement, 

 type of venation, and kind of leaf margin. Not one of these characters 

 is of any survival value to the plant, yet each of them is an external 

 expression of hereditary changes that occurred and have in some in- 

 stances survived for millions of years. 



When we distinguish families of plants on the basis of such differences 

 as the number of carpels in the pistil, the arrangement of the floral parts 

 in spirals or cycles, and the number of stamens and petals, we are again 



