xiii ORIGIN OF SPECIES 225 



though in all probability these organs have been secondarily 

 acquired as adaptations in connection with the habits of the 

 tadpole, and have therefore, unlike the gills, for instance, 

 no ancestral significance. 



It is obvious that the evolution of one species from 

 another presupposes the occurrence of variations in the 

 ancestral form. As we have seen, such individual variation 

 is of universal occurrence. This may be expressed by 

 saying that heredity, according to which" the offspring tends 

 to resemble the parent in essentials, is modified by varia- 

 bility, according to which the offspring tends to differ from 

 the parent in detail. If, from any cause, any well-marked 

 individual variation is perpetuated, there is produced what 

 is known as a variety of the species, and according to the 

 theory of the origin of species by evolution, such a variety 

 may, in course of time, become a new species. Thus a 

 variety is an incipient species, and a species is a (relatively) 

 permanent variety. 



One other important factor in evolution must be briefly 

 referred to before concluding this chapter. In order that 

 every species of animal and plant may flourish, certain 

 conditions are necessary. Thus the frog requires, for 

 example, a moist place to live in, and water in which to 

 lay its eggs. For spots presenting the necessary favourable 

 conditions, there is constantly going on a competition 

 between individuals of one species and between the mem- 

 bers of different species. The nature of this struggle is 

 well seen when a piece of garden ground is allowed to 

 run to waste. Its surface is soon overgrown by weeds of 

 many kinds, which kill out nearly all the original garden- 

 plants by depriving them of light and food. By and by the 

 more hardy weeds exterminate and replace such weaker 



PRACT. ZOOL. O 



