172 ANIMAL LIFE AND HUMAN PROGRESS 



chromosome is known as the X chromosome, and the bent 

 one is termed the Y chromosome. Normally, then, every 

 female has two X chromosomes, while every male has one X 

 and one Y chromosome. Or, to state it in another way, every 

 fly which receives an X chromosome from each parent is a 

 female, while the fly that receives an X chromosome from 

 its mother and a Y chromosome from its father is a male. 



Striking confirmation of this explanation has recently been 

 brought forward by Bridges. As the result of abnormal cell 

 divisions flies are occasionally produced which receive two 

 X chromosomes from their mother and a Y chromosome from 

 their father. Nevertheless the presence of the two X maternal 

 chromosomes decides their sexual fate. They are females in 

 spite of the male Y element. Other observations of the 

 highest interest in this connection have been made by Bridges, 

 to whose paper must be referred those who are interested in 

 these matters. For us here the outstanding feature of it all 

 is that a definite and not unsuccessful attempt has been made 

 to link up hereditary characters with definite and visible 

 structures in the cells of which animals and plants are 

 composed. 



If this line of work ultimately makes good it may bring 

 within the bounds of possibility the alteration of the hereditary 

 properties of germ cells by direct action through physical or 

 chemical means. The biologist of to-day inclines to the 

 view that new characters in plants or animals are generally 

 initiated in the cell divisions which give rise to the germ ceils. 

 Were the normal distribution of hereditary stuff at such 

 divisions disturbed, we should obtain new combinations, some 

 of which might very well be stable. If it be proved that the 

 chromosomes are the bearers of this hereditary stuff we are 

 clearly in a better position to appreciate the most favourable 

 moment to bring extraneous influences to bear. I have little 

 doubt that attempts will soon be made to bring about fresh 

 groupings of the chromosomes, whether by magnetic, electrical 

 or other means, and I am quite prepared to hear of such 

 attempts being attended with success. 



Secondary Sexual Characters. Sex itself is, after all, but a 



