ON PARASITES. 39 



quent history of the eggs, after impregnation is quite imperfect. 

 The eggs are doubtless cast into the outer world with the excrements, 

 and lie upon dnngheaps, ia cess pools, in pastures, &c., and as they 

 fall into conditions favorable for development, that process goes on 

 until finally, in the intestines of animals similar to those the parent 

 occupied before, they attain the state of the perfect individual. The 

 discoveries which have already been made, are sufficient to stimulate 

 to fresh exertions, and although the field of investigation is very 

 extensive, still a difficulty is a thing to be overcome. Among the para- 

 sites infesting marine animals, will be found the greatest difficulty 

 in unravelling the tangled threads of life, owing to the enormous 

 extent at which animal life is maintained, at the expense of animal 

 life, beneath the surface of the sea. When, as has happened within 

 the short space of an hour, four lives have been successively sacrificed 

 to maintain one, by the stronger and larger swallowing a weaker and 

 smaller one, and this in turn another still more defenceless, and so 

 on for four degrees, it is clear that any parasite which should happen 

 to infest the weakest of the prey, will have undergone a good many 

 vicissitudes, and found the conditions for its development greatly 

 complicated. Erom the fecundation of the egg, to the development 

 of the perfect individual, parasites of different species infesting ma- 

 rine animals are passing through their alimentary canals, and small 

 wonder would it be, if not a single egg ever came to maturity. But 

 so perfect is the adaptation of these creatures to the circumstances 

 in which they are placed, that enough of them pass unharmed 

 through the stages of development, to maintain perpetually the origi- 

 nal type, free from all danger of extinction. Similar, but in a less 

 degree, are the conditions upon land. The stronger individuals 

 always maintain themselves at the expense of the weaker, and as 

 they gratify their destructive propensities, render themselves the 

 prey of creatures still more defenceless, whose very insignificance 

 is their best security. Each animal under the motor influence of 

 its will, which in turn is stimulated by necessity, seeks to maintain 

 the requisite conditions, for its own existence at the expense of its 

 neighbours, and ia, as it were parasitic to them. Remove those con- 

 ditions, and at once the animal ceases to be. The entire world of 

 animal life is parasitic to itself, and to the vegetable, which, in turn, 

 depends upon the animal for its supplies, — mutual dependance binds 



