EFFECTS OF MIXING DIVERSE ORGANISMS 275 



tlon; by their union the hybrids may produce a few off- 

 spring. Such offspring may be abnormal or imperfect, or 

 some of them may develop normally. 



For example, Federley ^ crossed two species of butter- 

 flies, Pygaera curtula, which has 58 chromosomes, and Py- 

 gaera anchoreta, which has 60 chromosomes. The hybrids 

 (Fi) produced are normal and have 59 chromosomes; 29 

 from P curtula, 30 from P anchoreta. In forming germ cells 

 (sperms) in the hybrids, the curtula and anchoreta chro- 

 mosomes refuse to conjugate; or a few conjugate, the rest do 

 not. The result is that there is no reducing division; in the 

 final germ cells the full number of 59 chromosomes is pres- 

 ent. Add to this the fact that in attempting to conjugate 

 some of the chromosomes are injured or partly destroyed. 

 The result is that the sperms thus produced are abnormal. 

 Most of them will not fertilize the ova; they die without 

 giving origin to offspring. But a very few succeed in ferti- 

 lizing ova and producing a few offspring (F2). 



In another cross of butterflies studied by Federley, one 

 species, Pygaera pigra, has but 46 chromosomes, while the 

 other, Pygaera curtula, has 58. In the hybrids there are 

 23 -(- 29, or 52 chromosomes. In the formation of sperms 

 by the hybrids, most of the two kinds of chromosomes re- 

 fuse to conjugate. Usually some half-dozen conjugate, the 

 rest do not. Only those that conjugate undergo the reducing 

 division; the others divide, half of each going to each germ 

 cell. Thus the sperms have anywhere from 44 to 52 chro- 

 mosomes. On the other hand, in forming the ova, usually 

 the 23 pigra chromosomes conjugate with 23 of the curtula 

 chromosomes, leaving 6 curtula chromosomes that do not 

 conjugate. Very few offspring are produced by the hybrids. 



In cases in which the two parents have different numbers 

 of chromosomes, in the hybrids only a part of the chromo- 

 somes can conjugate, as in the example just described. A 

 well known example of this sort is found in the plant hy- 



