144 OUTLINES OF EVOLUTIONARY BIOLOGY 



is repeated again and again throughout the summer months and 

 thus the aphides multiply with great rapidity. In the autumn, 

 however, males and perfect females are again produced. The 

 viviparous imperfect females, as well as the males, are generally 

 winged, the perfect females are wingless. We have here another 

 kind of alternation of generations, in which forms which repro- 

 duce parthenogenetically alternate with others which exhibit 

 the normal sexual process ; to this type of alternation the 

 term heterogeny is sometimes applied. A very large number 

 of parthenogenetic generations may intervene between two 

 sexual ones. 



Another well-known case of parthenogenesis is that of the hive- 

 bee, where the eggs laid by the queen may either be fertilized or 

 not, in the former case giving rise to females (workers or queens) 

 and in the latter to males (drones). Other instances occur amongst 

 those parasitic flat-worms known as flukes (Trematoda) and in 

 some Crustacea (Cladocera, the so-called water fleas). 



The most remarkable cases of natural parthenogenesis, how- 

 ever, are those to which the special term paedogenesis has been 

 applied, in which the imperfect females do not even wait to 

 attain maturity before producing their offspring, but actually do 

 so in the larval condition, as in Chironomus and some other 

 two-winged flies (Diptera). 



In general we may say that parthenogenesis occurs in cases 

 where it is desirable to take advantage of a brief season of favour- 

 able conditions to multiply the race as rapidly as possible. It is 

 necessary to make hay while the sun shines. When adverse 

 conditions set in, such as the advent of winter in the case of the 

 aphides, or discharge from the body of the host in the case of 

 parasitic flukes, the vast majority of the race will perish, but 

 a sufficient number will be able to protect themselves in some 

 way (like the encysted cercariae of the fluke), or a sufficient 

 number of fertilized and protected eggs will be produced (as in 

 the aphides) to tide over the evil time and form the starting 

 points for fresh generations at the first favourable opportunity. 



It seems possible, also, that in some cases parthenogenesis 

 may be continued indefinitely without fertilization ever occurring, 

 for in certain species of minute rotifers and crustaceans no males 

 have as yet been observed. 



Recent researches have shown that parthenogenesis can be 

 artificially induced in cases where it does not occur naturally at 



