294 THE SIGNIFICANCE OF SEXUAL BEPBODUCTION 



In some Ostracodes, on the other hand, the males appear to be 

 entirely wanting- : at least, I have tried in vain for years to 

 discover them in any locality or at any time of the year 1 . 



Cypris vidua and Cypris reptans are such species. Now, although 

 the transformation of these formerly bisexual species into purely 

 unisexual female species appears to be complete 2 , yet the females 

 still possess a large, pear-shaped receptaculum 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 retrogressive changes could be 

 therefore easily detected. Furthermore among insects, in the genus 

 Chermes the receptaculum 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 3 . The case is quite different in species which 

 retain both sexual and parthenogenetic reproduction. Thus, the 

 summer females of the Aphidae have lost the receptaculum seminis ; 

 and in these insects sexual reproduction has not ceased, but alter- 

 nates regularly 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 im- 

 possible 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 



1 Compare my paper, ' Parthenogenese bei den Ostracoden,' in ' Zool. Anzeiger,' 

 1880, p. 82. Purely negative evidence, unless on an immense scale, is quite rightly 

 considered to be of no great value in most cases. But the condition of these animals 

 renders the accumulation of such evidence unusually easy, because the presence of 

 males in a colony of Ostracodes can be proved by a very simple indirect test. Thus 

 if a colony contains any males the receptacula seminis of all mature females are filled 

 with spermatozoa, and on the other hand we may be quite sure that males are 

 absent, if after the examination of many mature females, no spermatozoa can be 

 found in any of their receptacula. 



2 We cannot, however, be absolutely certain of this, for it ia conceivable that 

 males may still occur in colonies other than those examined. 



3 It has now been shown by Blochmann that males appear for a very short time 

 towards the close of summer, as in the case of Phylloxera. A.W., 1888. 



