INSECTS AIFF-CTINC 1>.\ U K AMI WOOUI.AX 1) TREES 249 



will thus be seen that inulti|)licatii)ii insteatl of hcint; arithmetic is really 

 jreometric, and this accounts for the innncnse nimiher of dcscenilants which 

 may result from one individual in a single season, {)rovided there is an ade- 

 quate food supph". This \ery rarely or never occurs in nature and it is 

 extremeh' fortunate that such is the case. The hoj) a|)his, to refer to this 

 species ao;ain, normally produces three generations on the [)lum, about eight 

 on the hop and then migrates back 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 exceedingly rapid. 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 vital energy in each species 

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

 development of efficient, aggressive weapons, for a substantial defensive 

 armor, or these may l)e sacrificed in turn for prolificacy and immense 

 numbers of comparativeh' defenseless individuals producetl. This latter is 

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

 tions of asexual or agamic individuals. The limit of asexual reproduction 

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

 vation for a period of four years and during that time failed to detect the 

 presence of a single male. Practically the same thing has been done bj 

 Professor Slingerlaml ' in later years with an aj^his, probabl\- Myzus 

 ac h )■ r a n t e s Monell, on Ach)ranthes (Iresine), and in this latter instance 

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

 several young on the day they were born and as soon as they produced 

 young their descendants were isolated and this continued uninterruptedly 

 for two years and ten months, during which the insect was carried through 

 62 generations. ;- ' ..„-',...,. ■ 



'1893. Science. Jan. 27. 21 : 48-49 



