Morgan arranges the numerous mutations observed in _Drosophila_ in four groups, corresponding in his opinion to the four pairs of chromosomes occurring in the cells of the insect. After the meiotic or reduction divisions each gamete of course contains in its nucleus four single chromosomes. One of the four pairs consists of the sex-chromosomes. All the factors of one group are contained in one chromosome, and it is found in experiments that the members of each group tend to be inherited together--that is to say, if two or more enter a cross together, in other words, if a specimen possessing two or more mutations is crossed with another in which they are absent, they tend to segregate as though they were a single factor. This fact agrees with the hypothesis that the factors in such a case are contained in a single chromosome which segregates from the fellow of its pair in the reduction divisions. Exceptions may occur, however, and these are explained by what is called 'crossing over.' When one chromosome of a pair, instead of being parallel to the other in the gametocyte, crosses it at a point of contact, then when the chromosomes separate, part of one chromosome remains connected with the part of the other on the same side and the two parts separate as a new chromosome, so that two factors originally in the same chromosome may thus come to lie in different chromosomes. In consequence of this, two or more factors which are usually 'coupled' or inherited together may come to appear in different individuals. Morgan emphasises the statement that a factor does not affect only one particular organ or part of the body. It may have a chief effect in one kind of organ, _e.g._ the wings or eyes, but usually affects several parts of the body. Thus the factor that causes rudimentary wings also produces sterility in females, general loss of vigour, and short hind legs.