122 . BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the offspring all possible combinations of the three kinds of chromo- 
somes. Or, if mating involving any two of the types could be made, 
there should result all possible combinations between them. On 
account of the small number of animals available for my study, no 
conclusions as to whether these conditions are realized in nature could 
be drawn. The presence of the three types in these few animals, 
however, strongly suggests the possibility of realization, especially 
since two of the three possible combinations are realized for the two 
kinds of chromosome in type Ci. The presence of a third type also 
suggests that there may exist in this case the mechanism for the 
transmission of triple allelomorphs. 
b. Some experimental Evidence. 
The most extensive breeding experiments the results of which tend 
to show that the chromosomes are concerned in the transmission of 
hereditary characters are those on Drosophila by Prof. T. H. Morgan 
and his students. In the course of this work they have dealt with 
over a hundred unit-characters which show Mendelian inheritance, 
either in a typical or modified form. In Drosophila, there are four 
pairs of chromosomes, of which one pair is very small, and one is a 
pair of heterochromosomes, or “sex-chromosomes.” In their behavior 
in inheritance, the hundred and more characters fall into four groups, 
each group tending to behave as a unit, just as it would be expected 
to do in case it were carried by a single pair of chromosomes. Of 
these groups of characters, one is very small, the others much larger, 
the largest one being the group of “sex-linked” characters. Naturally 
the small group of characters has been correlated with the small pair 
of chromosomes and the group of sex-linked characters with the sex- 
chromosomes. 
But there have been exceptions in the case of many pairs of allelo- 
morphs, especially those that are sex-linked, 7. e., cases where factors 
belonging to a certain group have gone into a mating together, but 
have not always reappeared together, as they would be expected to do 
if they were all carried by a single chromosome and that chromosome 
maintained its individuality. These phenomena have been explained 
by the so-called “cross-over” hypothesis. In this connection Morgan 
(11) developed what. has been termed the “linear arrangement”’ 
hypothesis, which was further elaborated by Sturtevant (13). These 
authors assume that the factors, or “genes,” which represent the 
