A DEFINITION OF EVOLUTION 



The Prodigality of Nature. The prodigality of nature with respect to 

 reproduction is well known. A single salmon produces 28,000,000 eggs in 

 a season; an oyster may pass as many as 114,000,000 eggs at a single 

 spawning; and Ascaris luinhricoides var. siium, a common parasite of hogs, 

 has been observed to pass as many as 700,000 eggs in a single 24-hour- 

 period under laboratory conditions. That such immense numbers of indi- 

 viduals should survive and themselves reproduce in similar numbers is 

 simply unimaginable. For example, a thorough study of a small sector of 

 the Pacific coast just north of San Francisco revealed about one hundred 

 starfish ( mainly Pisasfer ocraceus, but a few other species were included ) . 

 Assuming that half of these were females, and that each produced one 

 million eggs (a modest estimate), the population in the next year would 

 be about 50,000,000. These would include about 25,000,000 females, all 

 of which would again produce about a million eggs each. It is obvious 

 that, if the ordinary rate of reproduction were to continue for even a few 

 generations with 100 per cent survival of all offspring, soon the starfish 

 would fill the seas and be pushed out across the lands by sheer pressure 

 of reproduction. Indeed, at the rate of reproduction here described, it 

 would take only fifteen generations for the number of starfish to exceed 

 the estimated number of electrons in the visible universe ( 10'° )! 



The animals discussed above are intentionally chosen from among the 

 more prolific members of the Animal Kingdom. Essentially the same situ- 

 ation applies, however, to even the most slowly breeding animals. Frogs, 

 while generally regarded as quite prolific, produce at the most 20,000 

 eggs annually (the bullfrog Rana catcshciana). Most species of frogs pro- 

 duce fewer than 1,000 eggs annually, while a few (robber frogs, familv 

 Leptodactylidae ) mav lay as few as six eggs annually. Perhaps the most 

 slowly breeding of organisms is the elephant. Darwin calculated the re- 

 sults of a minimal rate of reproduction for this animal. Elephants have 

 a life span of about 100 years, with active breeding life from about thirty 

 to about ninety years of age. During this period a single female will prob- 

 ably bear no fewer than six young. If all of these young survived and con- 

 tinued to reproduce at the same rate, then after onlv 750 years, the 

 descendants of a single pair would number about 19,000,000. 



Thus, regardless of the rate of reproduction of a species, it is clear that 

 its numbers would soon become impossibly large if all survived and re- 

 produced, simplv because the rate of increase is geometric and the abun- 

 dant young try desperately to survive. Because of this, there must be a 

 struggle for existence, with tlu> majority of the participants losing. This 

 struggle may take manv forms, as the struggle of the individual to over- 

 come adverse environmental conditions such as cold or drought, or to 

 escape predators, or to obtain an ade(juate shar(> of a limittHl h)od sup})lv 

 for which there are many competitors. |])ar\\ in thought of \\\c struggle as 

 being most intense between members of the same species \\'hicli must 

 compete for identical requirements of life. 



The constancy of adult populations is not true to the degree that Dar- 

 win thought, for populations of wild species may vary tremendously from 

 year to year. However, they never approach the si/.c^ calculated from the 



