656 LECTURE XXIII. 



descendants until a favourable variation is seized upon by 

 natural selection. As the female cell which produced the 

 organism thus selected will transmit the same variation by 

 direct inheritance to the female cells which it itself produces, 

 the characteristic will be established as an hereditary charac- 

 teristic, and will be perpetuated and transmitted, by the 

 selected individuals and their descendants, without gemmules. 

 This restatement certainly gets over the difficulty which it 

 was framed to meet, but it raises new difficulties. Without 

 contesting the possibility of cells throwing off gemmules, 

 it may be enquired why it is that gemmules should only 

 be thrown off under the stimulus of unfavourable conditions, 

 and how it is that the reproduction of those parts which 

 do not throw off gemmules is effected. Darwin's theory is 

 at least consistent in that it applies to the reproduction of 

 all parts whatsoever, whereas Brooks' theory is not. The 

 restatement is certainly not less open to objection than the 

 original theory. 



In connexion with his restatement of the theory of Pange- 

 nesis, Brooks proposes a theory of the sexual process which 

 may be mentioned here, but which will not be discussed until 

 later. It is generally assumed that both the male and the 

 female cells transmit to the offspring of their union the 

 characters of the two individuals which have produced them 

 respectively. Brooks, however, considers that the function 

 of the two cells in relation to the progeny is different. 

 According to his view the male cell is the originating, and 

 the female the perpetuating, factor; the ovum is conservative, 

 the male cell progressive. Heredity, or adherence to type, 

 is brought about by the female cell; variation and adaptation 

 through the male cell; the female cell is the essential, the 

 male cell the secondary, factor in heredity. 



The next theory of reproduction which we will consider is 

 that put forward by Naegeli. Before entering upon the 

 discussion of it, a short account must, for the sake of clear- 

 ness, be given of his views as to the constitution of proto- 

 plasm. He distinguishes in protoplasm two parts, the fluid 

 (Hygroplasma) and the solid (Stereoplasma). The active 



