58 



but also complex in its structure, and 

 the seed production is prodigious. 



A general parallel may be drawn 

 between such parasitism in plants and 

 that in animals. The general similarity 

 of the effect of the habit upon organ- 

 isms so diverse as Rafflesia and a tape- 

 worm or a liver-fluke may be held as 

 demonstrating the biological principles 

 that underlie it in either kingdom. In 

 both the phenomenon is sporadic, a 

 fact which indicates that it arises in re- 

 lation to opportunity rather than by 

 anv definite evolutionary trend. Para- 

 sitism brings with it the easy acquire- 

 ment of food without the obligation of 

 gathering or acquiring it by individual 

 exertion. So far it is a direct advantage 

 to the organism that adopts it. But the 

 habit brings also two natural conse- 

 quences. The first is a reduction or even 

 abortion of parts. Tlie disused organs— 

 and in particular the leaves and roots of 

 plant parasites— not being necessar\', 

 are partially or wholly aborted; while in 

 flowering plants developed as parasites 

 the whole vegetative system may in ex- 

 treme eases be represented by filaments 

 no more elaborate than are the fila- 

 ments of a fungus. The second conse- 

 quence is an enormous production of 

 seeds. Any parasite that has wholly de- 

 sisted from self-nourishment stakes all 



ECOLOGY 



its chances of existence upon finding 

 the peculiar conditions of its supply. 

 Tlie chances of its doing so are the 

 more remote the more peculiar they 

 are. The germs of a parasite that is 

 successful only on one host must find 

 that host or perish. The risk of not do- 

 ing so is met by the immense output of 

 seeds, which is specially marked where 

 the parasitism is of an advanced type. 

 It is seen in the large seed-output of 

 the tooth-wort, which is parasitic on 

 hazel roots, or of Rafflesia parasitic on 

 those of certain vines. If we turn from 

 such examples to animal parasites, we 

 shall see in the liver-fluke and in the 

 tape-worm conditions of simplified 

 or indeed degraded bodily structure, 

 and of high fecundity, which may be 

 read as similar responses to biological 

 conditions that rule for parasitism in 

 either kingdom. They provide indeed 

 some of the widest possible examples of 

 homoplasy: that is, the similar reaction 

 of unrelated organisms to like condi- 

 tions of life. It appears, then, that the 

 result of parasitism may be stated quite 

 generally for any living type, including 

 even man himself: once the individual 

 reaches maturity, physiological de- 

 pendence and degradation tend to go 

 hand in hand. 



QUESTIONS 



1. Do scientists generally believe that 

 aborted organs arise because the organ 

 was not needed? Explain. 



2. Does the theory of Bovver sound like 

 Darwin's, Lamarck's or DeVries' 

 Theory? 



