302 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



seminis, with its long spirally twisted duct, which is surrounded 

 by a thick glandular layer. This is the more remarkable as the 

 apparatus is very complicated in the Ostracodes, and retrogres- 

 sive changes could be therefore easily detected. Furthermore 

 among msects, in the genus Chermes the rcccptaculuni seminis of 

 the females has also remained unreduced, although the males 

 appear to be entirely wanting, or at least have never been 

 found, in spite of the united efforts of several acute observers ^ 

 The case is quite different in species which retain both sexual 

 and parthenogenetic reproduction. Thus, the summer females 

 of the Aphidae have lost the receptaciihim scminis\ and in these 

 insects sexual reproduction has not ceased, but alternates regu- 

 larly with parthenogenetic reproduction. 



Certainly this proof of the truth of my theory as to the signi- 

 ficance of sexual reproduction is far from settling the question : 

 it only renders the theory highly probable. At present it is 

 impossible to do more than this, because we do not yet possess 

 a sufficient number of facts, for many of them could not have 

 been sought for until after the theory had been suggested. We 

 are here concerned with complicated phenomena, into which 

 we cannot acquire an immediate insight, but can only attain it 

 gradually. 



But, nevertheless, I hope to have shown that the theory of 

 natural selection is by no means incompatible with the theory 

 of * the continuity of the germ-plasm ; ' and, further, that if we 

 accept this latter theory, sexual reproduction appears in an 

 entirely new light : it has received a meaning, and has to a 

 certain extent become intelligible. 



The time in which men believed that science could be ad- 

 vanced by the mere collection of facts has long passed away : 

 we know that it is not necessary to accumulate a vast number 

 of miscellaneous facts, or to make as it were a catalogue of 

 them ; but we know that it is necessary to establish facts 

 which, when grouped together in the light of a theory, will 

 enable us to acquire a certain degree of insight into some 

 natural phenomenon. In order to direct our attention to those 

 new facts which are of immediate importance, it is absolutely 



' It has now been shown by Blochmann that males appear for a very 

 short time towards the close of summer, as in the case oi Phylloxaa. — 

 A.W., 1888. 



