MODIFICATION OF SPECIES 365 



numerous because it lays eggs rapidly. It is the totality of qualities, 

 some favorable, some unfavorable, that determines success, and it is on 

 this total product that selection acts. 



Another point requiring emphasis is that, from the evolutionary 

 standpoint, a successful species or individual is one that leaves many 

 descendants. No quality is of any particular advantage to a species 

 unless it entails numerous posterity. Long life may seem to be an advan- 

 tage; but if it is merely a prolongation of activity after the reproductive 

 period is over, the species gains nothing by it. Rapid growth is a 

 good sign physiologically; but if it is expressed only in somatic tissue 

 and does not result in more germ cells or more embryos or does not in 

 some way enlarge families, it is useless as an element in the security of 

 the species. On the whole, also, it is the far distant progeny, rather 

 than the near generations, which are most important. A species so 

 constituted that in its present environment it succeeds moderately but 

 safely, but will in a much later environment thrive exceptionally, is 

 more influential upon evolution than is a species which is exceedingly 

 abundant now but dwindles in later time. These statements are, of 

 course, merely definitions ; it is not possible to apply them and say which 

 present species are going to be successful later. 



Isolation. — Many biologists have always believed that an important 

 part of the divergence of species from one another is due to some sort of 

 isolation. Attention was early called to the supposed effects of geo- 

 graphic isolation, as of terrestrial organisms living on an island. The 

 species in such an isolated .region are mostly different from those of the 

 nearest other land areas. It is easy to see how, with different mutations 

 happening to arise among island forms, and probably with a different 

 sort of environment acting selectively upon them, there should be a 

 gradual divergence of the two groups. Taxonomists, moreover, have 

 generally held that the classification implies much more isolation than 

 geographic features provide. Since species have presumably split up 

 into varieties, which are free to cross with one another as far as they 

 meet in the same area, and since by further divergence varieties are 

 believed to advance to the rank of species, it might be supposed that 

 hybridization between species would continue indefinitely. Now hybridi- 

 zation should operate to remove the distinctions between species. How, 

 then, have arisen the generally rather sharp lines between species? For 

 there are relatively few intermediate individuals that might be regarded 

 as species hybrids. 



The nature of the answer to this question is indicated by the discovery 

 that most species are not fully fertile with other species, even with those 

 most like them. While there are many exceptions, especially in plants, 

 they are in a small minority. Some species, exen within the same 



