^he Trend of Evolutionary Thought. 9 



evolutio or distribution of these determinants. Variation, 

 according to him, is brought about by the mingling of germ- 

 plasms. 



The work of Gregor Mendel, published in 1865, but lost 

 sight of till it was confirmed by three independent workers in 

 1900, was of supreme importance in connection with Weismann's 

 theory. Mendel worked with peas, and showed how characters, 

 such as yellowness or greenness, tallness or smallness, do not 

 blend, but are alternative in inheritance ; and he further proved 

 that such characters are represented singly in the germ-cells.* 

 The great fact that he established, and that has been confirmed 

 time after time, alike for plants and for animals, is that there arc 

 definite representatives (which we may call " determinants ") of 

 certain unit characters in the germ-plasm. His work, then, may 

 be taken as so far confirmatory of the Weismannic theory of the 

 structure of the germ-plasm. It has thrown light, too, on the 

 formation of new species Ijy combination of characters or by the 

 dropping out of characters. 



To be associated with the Mendelian view of inheritance is 

 the recent work on heredity of sex. From this it appears that 

 there is good evidence that in some insects one particular 

 chromosome of the germ nucleus can be recognised as "sex 

 determining " in character. In the bug Protenor, for example, 

 the cells of the male have thirteen chromosomes, while those of 

 the female have fourteen, and one of the thirteen is larger than 

 the other twelve, and can be identified as " sex determining." 

 As a result of the usual maturation processes the male germ-cells 

 yield two types of speimatozoa, one with six chromosomes, all 

 alike, and the other with seven chromosomes, six small and one 

 large. The mature egg-cells, on the other hand, have all seven 

 chromosomes, six small and one large. If a spermatozoon Avith 

 six chromosomes fertilises an egg-cell, the result is a male with 

 thirteen chromosomes in its cells. But if a spermatozoon with 

 seven chromosomes fertilises an egg-cell, then the consequence is 

 * See " Mendel's Principles of Heredity," by Professor W. Batesoii. 



