IXSECTS AFFECTING PARK AM) WOODLAND TREES 249 



will thus be seen that multiplication instead of hein^- arithmetic is really 

 (geometric, and this accounts for the immense number of descemlants which 

 may result from one indi\-idual in a sing;le season, provided there is an aile- 

 quate food supply. 1 liis very rarel\- or never occurs in nature and it is 

 extremely fortunate that such is the case. The hop aphis, to refer to this 

 species again, normally proiluces three generations on the [lUmi, about eight 

 on the ho]) and then migrates Ijack to its original food plant, so that the 

 production of lo generations in a single season is normal for certain species. 

 The immense rapidity with which the life cycle is completed is rendered 

 possible by the development of generation after generation of agamic or 

 asexual individuals. This means that there is a nearly continuous produc- 

 tion of females, each of which begins producing young very early in life, so 

 that reproduction is exceedingh* rapitl. It is interesting- to note in this 

 connection that there appears to be some relation existing between the 

 defensive powers of an insect and its prolificacy. There is apparently, if 

 one may use the term, a certain amount of \ital energy in each species 

 which may be used in several ways. Prolificacy may be sacrificed for the 

 development of efficient, aggressive weapons, for a substantial defensive 

 armor, or these may be sacrificed in turn for prolificac)- ami immense 

 numbers of comparatively defenseless individuals produced. This latter is 

 true of plant lice and is accomplished mainly through successive genera- 

 tions of asexual or agamic intli\idimls. Tin- limit of asexual reproduction 

 has never been reached. Kyber in 1815 had the rose aphis under obser- 

 vation for a period of four )'ears and eluring that timt; failed to cletcxt the 

 presence of a single male. I'racticall)' the same thing has been dune b\ 

 Professor Slingerland ' in later years with an aphis, probably Myzus 

 achyrantes Monell, on Achyranthes (Iresine), and in this latter instance 

 there was no possibility for doubt, since the experimenter carefully isolated 

 several young on the day the)- were born anil as soon as they produced 

 young their descendants were isolateil and this continued uninterruptedly 

 for two )'ears and ten months, tluring which tin- insect was carried through 

 62 generations. 



■1893. Science. Jan. 27. 21 148-49 



