xra ORIGIN OF SPECIES 225 



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 varia- 

 tion is of universal occurrence. This may be expressed 

 by saying that heredity, according to which the offspring 

 tends tc resemble the parent in essentials, is modified by 

 variability, 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 tn"e~0rigin 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 members 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 ail the original garden-plants by depriving them 

 of light and food. By and by the more hardy weeds 

 exterminate and replace such weaker forms as may first 



PRACT. ZOOL. 



