ECOLOGICAL 153 



Nematodes, such as a common intestinal parasite of man in Tropical 

 Asia {Strongyloides ster cor alls) . 



(6) Deserving consideration by itself is the prolific multiplication. 

 Leuckart's estimates have been often quoted, that Tcenia solium 

 may produce 42 million eggs in a year, and an Ascaris 64 millions. 

 It is possible, no doubt, to discover some free-living animals still 

 more prolific, such as a starfish {Luidia ciliaris), which Mortensen 

 credits with containing 200 million eggs, but there are not many 

 instances of such extraordinary fecundity; not even among fishes. 

 Moreover, it is possible to compare the huge number of eggs produced 

 by many a Trematode, the liver-fluke's being estimated at 50,000, 

 with the small number produced by a not very distantly related 

 free-living Turbellarian of the same size. 



The eggs and larvae of parasites are often subject to severe elimi- 

 nation ; the chances of death are enormous. Therefore, in the course of 

 the evolution of parasites, variants in the direction of increased 

 productivity would have survival value. Thus the race now con- 

 tinues with a large margin. It may also be granted for some cases 

 that the conditions of the individual life, e.g. rich and abundant food, 

 will favour prolific reproduction. 



SOME EVOLUTION PROBLEMS.— (i) It must not be facilely 

 supposed that the adaptive peculiarities of parasites illustrate 

 individual modifications that have hereditarily accumulated until 

 they have become racial characters. Organisms that have begun the 

 parasitic mode of life because of certain constitutional weaknesses 

 may continue to show congruent germinal variations, some of which 

 may have selective value. (2) It must not be too confidently assumed 

 that all the diagnostic peculiarities of parasites are as such engrained 

 hereditary characters. Many of them may be individual structural 

 reactions to the peculiar conditions of life, which recur with each 

 generation. An organism's characters develop as the outcome of 

 environmental, nutritional, and functional nurture operating upon 

 hereditary nature. There is much to be said for the view that the 

 pigmy male of Bonellia suffers arrest of development partly because 

 it absorbs the secretion on the proboscis of the female. More attention 

 should be paid to phenomena like those of "physogastry" in the 

 guests of the white ants, where extraordinary deformations of body 

 come about in probably direct individual reaction to the unwhole- 

 some conditions of life. Experimental inquiry is also needed to show 

 whether some of the specific characters of nearly related parasites 

 may not be modificational. To test this experimentally, nearly 

 related species in different hosts should be exchanged in early life. 

 (3) It may be noted that many parasites probably illustrate the 

 results of "isolation", which narrows the range of inter-crossing. 

 For not only is the host in some ways Uke an island, to take the 



