56] CHAP. IV. SPECIAL PHYSIOLOGY OF REPRODUCTION. 231 



that they differ widely, since the former cannot (except in 

 rare cases), whilst the latter can, develope -by itself into a new 

 organism. 



The second question is as to the nature of sex : what is the 

 difference, if any, between a male and a female gamete ? In 

 some cases there is a marked external difference ; for instance, in 

 the Pteridophyta, Bryophyta, and many Algae, the female gamete- 

 is a large motionless oosphere, whilst the male gamete is a small- 

 actively-swimming sperrnatozoid. But this marked difference is 

 not essential, it is merely adaptive ; it is an adaptation to a more 

 or less aquatic mode of life or, at least, of fertilisation. Moreover, 

 it is obviously inapplicable in explanation of the many cases in 

 which the two conjugating gametes are externally quite similar. 

 Nor has minute microscopic investigation brought to light any 

 distinguishing criterion. But it must not be concluded on this 

 account that there is no difference between a male and a female 

 gamete ; it is obvious that there is an essential physiological' 

 difference between them. For, were it otherwise, it would be- 

 impossible to account for such a fact, for instance, as that even 

 where, as in many Algae, the gametes are all extruded into the- 

 water, fusion never takes place between two male or two female- 

 gametes, but only between a male and a female. 



Brief allusion may be made to the means by which the sexual' 

 process is ensured. It might be thought that the most effectual' 

 means would be the development of the male and female organs- 

 in close propinquity on the same individual. No doubt this is- 

 the case, but the result is to ensure the less advantageous mode 

 of the process, the mode of self-fertilisation ; in fact, in many 

 cases in which the male and female organs are thus developed close 

 together, as in moncBcious plants (p. 61), self-fertilisation is pre- 

 vented by the male and female organs maturing at different times. 

 The real problem is, then, to ensure a sexual process between two 

 gametes derived from distinct individuals. The end is attained 

 either directly, by bringing the diverse gametes together ; or 

 indirectly, by bringing the spores together, and consequently also 

 the gametophytes. 



The method of bringing the two gametes together is essenti- 

 ally connected with the aquatic mode of fertilisation. It has 

 been observed and investigated in plants in which, whilst the 

 oosphere is motionless and remains in the female organ, the 

 spermatozoids are free-swimming ; and it is among the most 



