6 4 



ZOOLOGY 



Mutations 

 with altered 

 numbers of 

 chromo- 

 somes 



Maturation 

 of germ 

 cells ; the 

 reduction 

 division 



only two, while other animals and plants have very 

 many. So regularly is any difference in the number of 

 chromosomes associated with a difference of species, 

 that naturalists have come to regard it as a specific char- 

 acter. Thus Professor E. B. Wilson was examining the ' 

 chromosomes of certain bugs (Hemiptera) which a 

 specialist in entomology had declared to be all of one 

 species. He discovered that he had two lots, from dif- 

 ferent parts of the country, differing in the number of 

 chromosomes. He then returned some of them to the 

 specialist, who was able to find valid external characters 

 in the insects, and was obliged to confess that he had 

 been mistaken ; that there really were two species. 

 There are cases, however, in which differences in the 

 chromosome number have arisen under observation, and 

 do not separate what we should ordinarily call species. 

 Lamarck's evening primrose has 14 chromosomes; but 

 from it has arisen a large form, called by De Vries 

 (Enothera gigas, which has 28 chromosomes. Another, 

 called semigigas, has 21. These plants, which De 

 Vries calls mutations, have very distinct external char- 

 acteristics accompanying the difference in chromosomes. 

 If we found them on different islands or in different 

 countries, knowing nothing of their history, we should 

 doubtless call them "good species," and should point 

 to the chromosome count in confirmation of our opinion. 

 4. The behavior of the chromosomes in the germ 

 cells or fertilizing cells is remarkable, and confirms the 

 view that they are of particular significance for heredity. 

 The germ-cell material is in most cases set aside early 

 in development, and is compelled to wait until sexual 

 maturity for its opportunity. It does not become 

 specialized, in the manner of muscle cells or nerve cells, 

 as by so doing it would lose the power of contributing 



