230 EN TO-PARA SITES AND HYGIENE. 



ready to understand the law that the young never grow- 

 up beside their parents, at least never attain their full 

 development, but must always leave their abode and seek 

 a new host before they can reproduce their species. 

 This seems to be rather beneficial to the host, but it is a 

 somewhat precarious state for the parasites whose exist- 

 ence is now to a large extent left to chance, because, as 

 said before, the parents cannot provide a way for their 

 young to get along in the world, but must deposit them 

 at random. This danger is met on the side of the para- 

 sites by the production of an enormous number of germs, 

 so that in every place and at all times the opportunity is 

 given to infect a suitable animal happening to be near 

 by. All parasites have an enormous fertility, a fertility 

 which leaves anything else in this line far behind. 

 Many such parasites are, strictly speaking, nothing but 

 living ovaria, whose other organs have more or less com- 

 pletely disappeared in favor of the egg-producing or- 

 gans. Taenia solium, for instance, produces 42 million 

 eggs yearly; ascaris lumbricoides, 64 million eggs, 

 which would be the same thing as if the human female 

 gave birth to seventy children every day throughout the 

 year. 



This immense fertility is easily conceivable when we 

 imagine that no other animals find all the conditions for 

 it so favorable. They need not spend any time nor 

 bodily effort on locomotion nor the supplying and diges- 

 tion of food; they can easily turn all their energies upon 

 reproducing their species. Thus we see that these two 

 circumstances, which are important for the existence and 

 preservation of the animals, are also in close connection 

 with each other. Gfreat fertility enables the parasites to 

 risk a rather uncertain way of reproduction, i. e., of pres- 

 ervation of species, and their mode of living warrants on 

 the other side an extreme fertility, an enormous number 

 of progeny. There is never a lack of probability that 



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