6/6 LECTURE XXIII. 



cultivated plants rarely remain exposed to closely similar 

 conditions during any considerable length of time. Again, 

 sexual reproduction, especially in connexion with cross-ferti- 

 lisation, increases variability. It appears that greater varia- 

 bility, like greater vigour and fertility, is one of the advan- 

 tages gained by means of sexual reproduction. 



It appears probable, from the foregoing considerations, 

 that variability was first induced as the response of the 

 organism to changes in the conditions of life, and that it 

 has become intensified by sexual reproduction and by culti- 

 vation. We know, from such observations as those of Kol- 

 reuter and Gartner, who found that when two species were 

 crossed, if either one was variable, the offspring were variable, 

 that variability is hereditary ; and we may therefore conclude 

 that variability has come to be a general property of proto- 

 plasm. We may, in fact regard variability as a form of 

 irritability, which, like all other forms of irritability, originated 

 in the action of external stimuli and has become permanent. 



We will digress for a moment to consider Brooks' view 

 that variability is especially transmitted by the male cell. 

 In support of this view he adduces the fact, ascertained 

 by Gartner and Wichura, that the progeny of a pure species 

 crossed with a hybrid as the father are more variable than 

 the progeny of the hybrid fertilised by the pollen of the pure 

 species. It must be borne in mind, however, that, as just 

 mentioned, there are facts which indicate that variability may 

 be transmitted by either sex. Further, this particular case 

 will not establish Brooks' position unless it be proved that 

 the variability of the hybrid is transmitted by its male cell in 

 a higher degree than its other characters. We know, namely, 

 that the characters of an individual are frequently more 

 readily transmitted by the one cell than by the other. This 

 may be especially so in the case of hybrids ; it may be that 

 in hybrids there is a peculiar difference in potency between 

 the male and female cells, and that all the characters, and 

 not only the variability, of the hybrid, are transmitted more 

 perfectly by the male than by the female cell. The difficulty 

 in determining this point lies in the high variability of the 



