148 LIFE : OUTLINES OF GENERAL BIOLOGY 



The vegetative body of Rafflesia may be reasonably called 

 degenerate, but it must be asked, following MacGregor Skene, 

 whether all the simplifications of parasitic plants deserve this term, 

 and whether the simplifications are to be regarded as primarily 

 associated with the parasitism. The beginnings of reduction, e.g. 

 in leaves and in amount of chlorophyll, are often to be seen quite 

 apart from parasitism. Some reduction in assimilatory power, or 

 some other inferiority in competing with rivals in crowded conditions, 

 may prompt root-parasitism or some other parasitic dependence. 

 And when the habit of parasitism or partial parasitism has begun, 

 it is not unreasonable to suppose that degenerative variations, e.g. 

 towards "golden leaves" or towards albinism, would be more likely 

 to survive because of the abundant nourishment supplied by the 

 host. 



ORIGIN OF PARASITISM.— (a) Many animal parasites, such as 

 some of the Nematodes, may have begun as saprophytes. Several 

 flies that normally lay their eggs in putrefying animals may simi- 

 larly utilise the abrased skin of one that is still living. (6) Many 

 animals are cryptozoic, given to hiding themselves, or negatively 

 heliotropic, inclined to seek out narrow and shaded passages; and 

 this is another way in which parasitism may arise. The common- 

 place is often overlooked that the host is not to the parasite another 

 organism, for that would mean an incredibly subtle awareness of 

 the situation, but merely a convenient and attractive environment. 

 (c) It is likely that parasitism often arose when the struggle for 

 existence was extremely keen, when even slightly open doors were 

 welcome. As already indicated, the most characteristic feature in 

 parasitism is probably some weakness or passivity of constitution, 

 a "disposition" more inclined to drift than swim. Just as some such 

 animals became cave-dwellers, others became parasitic, (d) Crusta- 

 cean parasites are particularly interesting because they afford 

 many illustrations of parasitism restricted to the females; in whom, 

 in addition to the passivity frequently associated with the sex, 

 the parasitic habit might arise in connection with the advantage of 

 securing sheltered nooks for liberating eggs or offspring, {e) Where 

 sex dimorphism in growth—increased in the female and diminished 

 in the male- becomes very pronounced, as in Bonellia, many 

 Coj^^pods, also, and even two mid-water Angler-fish, there would 

 be an advantage to the survival of the tiny males, and even to the 

 females themselves, in this degeneration of males, since securing 

 fertilisation, and so maintaining the species. In the Anglers alluded 

 to. which occur sparsely in thinly peopled waters, the female 

 carries the minute male ; and all his nourishment is derived from an 

 organic connection between his head and the blood-vessels of some 

 part of her skin. In the extreme sex dimorphism of many Rotifers, 



