CHAPTER VIII 



THE TETRAPLOIDS, OR FOURFOLD TYPE- 



THE chromosomes have been counted in more than 

 a thousand species of animals and probably in as 

 many or more species of plants. In two or three 

 species only one pair of chromosomes is present. At the 

 other extreme there are species with over one hundred 

 chromosomes. No matter how many or how few the chro- 

 mosomes, the number is found to be constant for each 

 species. 



It is true that irregularities in the distribution of the 

 chromosomes occasionally take place. Most of these ir- 

 regularities are, as a rule, automatically straightened out 

 in one or another way. It is also true that, in one or two 

 cases, a slightly variable number of chromosomes may 

 occur, as in Metapodius where one or more small, extra 

 chromosomes, sometimes the Y-chromosomes, sometimes 

 another chromosome called the M-chromosome, mav or 

 may not be present (Fig. 62). As Wilson has shown, these 

 chromosomes may, perhaps, be looked upon as indifferent 

 bodies that have lost their importance, since there are no 

 corresponding variations in the characters of the indi- 

 viduals. 



It is known, furthermore, that chromosomes may join 

 together, decreasing the number by one or more, but the 

 totality of the genes is still preserved, and this also holds 

 for cases where a chromosome may break, increasing for 

 a time at least the number by one. 1 Finally, there are 



1 The occasional breaking apart of chromosomes in Oenothera has been 

 described by Hance. In the moth PhragmO(tobia, and in other moths also, 

 Seiler has described several cases where certain chromosomes that are united 



CO 



