33^ SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



favourable, in order that the gemmules which produce variation 

 may be thrown off, it is obvious that such a result would not 

 occur in the case mentioned above. Suppose, for example, 

 that the spines of a hedgehog are not sufficiently long or 

 sharply pointed to afford protection to the animal, how could 

 such an unfavourable development aftbrd the occasion for the 

 throwmg off of gemmules, and a resulting variability of the 

 spines, inasmuch as the epidermic tissue in which these struc- 

 tures arise, remains under completely normal and favourable 

 conditions, whatever length or sharpness the spines may attain ? 

 The conditions of the epidermis are not unfavourably affected 

 because, as the result of short and blunt spines, the number of 

 hedgehogs is reduced to far below the average. Or consider the 

 case of a brown caterpillar which would gain great advantage 

 by becoming green ; what reason is there for believing that the 

 cells of the skin are placed in unfavourable conditions, because, 

 in consequence of the brown colour, far more caterpillars are 

 detected by their enemies, than would have been the case if 

 the colour were green ? And the case is the same with all 

 adaptations. Harmony between the parts of the organism is 

 an essential condition for the existence of the individual. If it 

 is wanting, the individual is doomed ; but such harmony be- 

 tween any one part and all others, i. e. proper nutrition for 

 each part, and adequate performance of its proper function, 

 can never be disturbed by the fact that the part in question is 

 insufficiently adapted to the outer conditions of life. According 

 to Darwin, all the cells of the body are continually throwing off 

 gemmules, and against such an assumption no similar objection 

 can be raised. It can only be objected that the assumption has 

 never been proved, and that it is extremely improbable. 



A further essential difference between Darwin's theory of 

 pangenesis and Brooks' hypothesis lies in the fact that Brooks 

 holds that the male and female germ-cells play a different part, 

 and that they tend to become charged with gemmules in dif- 

 ferent degrees, the egg-cell containing a far smaller number 

 than the sperm-cell. According to Brooks the egg-cell is the 

 conservative principle which brings about the permanent trans- 

 mission of the true characters of the race or species, while he 

 believes that the sperm-cell is the progressive principle which 

 causes variation. 



