^o8 GENERAL BIOLOGY 



Sometimes they are found side by side, as are the long winged 

 and short winged crickets of the Eastern States, or, they may 

 be related to season, as are three forms of the Ajax butterfly. 

 Species that have been long domesticated always show 

 greater diversity, due to man's influence in selecting and 

 isolating the most divergent types — especially such types as 

 natural selection would ruthlessly eliminate. This is the 

 only way of obtaining new forms. We cannot compel 

 nature to produce anything ; we can only wait upon her, and 

 preserve our choice of what she offers, from the swamping 

 effects of intercrossing and from the rigors of a harsh environ- 

 ment. When the breeder of plants or animals wishes to 

 obtain a new strain, he breeds together forms that differ 

 with respect to the characters of which he desires to secure a 

 modification. This is hybridization. If all the offspring 

 of any given variety that is bred inter se, are like in any given 

 character, they may for all practical purposes be considered 

 in respect to that character pure bred. 



Types of inheritance.— What the result will be when any 

 two varieties are crossed, what characters the offspring will 

 bear, can only be determined by trial ; it will always be the 

 same between the same two pure varieties. Observations 

 on matters of this sort fall outside the possible scope of the 

 practical studies of this course; we will content ourselves, 

 therefore, by noting in passing a few of the more general 

 phenomena of hybridization. 



As to heritability of results, the offspring may be sterile, 

 and therefore, self-annihilating. The best known example 

 is the mule, a cross between the horse and the ass. There is 

 no race of mules; other mules are to be obtained only by 

 repeating the crossing of the parent species. Or, the hy- 

 brids may be relatively stable, and breed true, as single new- 

 formed race from the beginning. The garden sunberry, a 

 cross between two wild inedible species of Solanum, is said 



