358 PRINCIPLES OF ANIMAL BIOLOGY 



possible combinations is doubled. The importance of this sort of varia- 

 bility in evolution can scarcely be overestimated. A species that is 

 confronted by a number of environmental situations may easily be in a 

 position to take advantage of several of them. Its success would thereby 

 be enhanced. 



The variability that is due to combinations of genes in different ways 

 is changed in its nature when genes of different pairs interact with one 

 another to produce a character not like that produced by either one 

 alone. An example is given on page 232 and in Fig. 201, where walnut 

 comb is produced in fowls by a combination of the genes for pea and rose 

 comb. The genes for brown and scarlet eye in Drosophila produce 

 together a nearly white eye. Many such interactions between genes are 

 known. It is indeed doubtful whether any gene fails to interact with 

 those of other pairs in some way. Such interactions do not increase the 

 number of different kinds of individuals which may result from recombi- 

 nation of genes, but they do introduce unpredictable qualities into the 

 species. This feature may likewise be highly important to a species in a 

 variable environment. 



The hybridization referred to alcove h merely that occurring between 

 slightly unlike individuals within the species. Whether hybridization 

 occurs between two species or not depends partly on whether their 

 chromosomes are similar and equally numerous. If the species has the 

 same number of chromosomes and if the genes in them are in large 

 measure alike, crossing is usually possible. The normal pairing of the 

 chromosomes in the preparatory stages of germ cells depends on these two 

 things. If the numbers of chromosomes are not equal, odd single chromo- 

 somes are left over from this pairing. And if corresponding genes do not 

 exist in l)oth species, the chromosomes do not unite readily. Many 

 abnormalities result from these situations. The majority of species 

 crosses fail to produce offspring, or the offspring are partially or wholly 

 sterile. It seems unlikely, therefore, that any considerable part of evolu- 

 tion is due to hybridization between species. 



The Direction of Evolution. — Evolution has taken by no means all 

 of the courses that were theoretically open to it. Even if life originated 

 only once, and even though the million or two species now probably in 

 existence is a good round number of end products, this degree of differen- 

 tiation is much less than might conceivably have occurred. The actual 

 divergence of lines of descent has been considerably curtailed. What the 

 other possibilities were that have not been realized, why certain species 

 were produced and not others, why certain spcnnes that were produced 

 survived and not others are problems to which we must now turn. Their 

 solution is largely speculative but impoi'tant. 



The first element entering into the direction of evolution is the charac- 



