INFUSORIAL ANIMALCULES. 529 



perfect insects similar to their parents. The difficulty of account- 

 ing for the presence of the maggots by generation, therefore, de- 

 pends simply on the fact that they are different in appearance from 

 the parents that produce them. This difference, however, is merely 

 a temporary one, corresponding with the difference in age, and dis- 

 appears when the development of the animal is complete; just as 

 the young chicken, when recently hatched, has a different form and 

 plumage from those which it presents in its adult condition. 



Nearly all the causes of error, in fact, which have suggested at 

 various times the doctrine of spontaneous generation, have been 

 derived from these two sources. First, the ready transportation of 

 eggs or germs, and their rapid hatching under favorable circum- 

 stances ; and secondly, the different appearances presented by the 

 same animal at different ages, in consequence of which the youthful 

 animal may be mistaken, by an ignorant observer, for an entirely 

 different species. These sources of error are, however, so readily 

 detected, as a general rule, by scientific investigation, that it is 

 hardly necessary to point out the particular instances in which they 

 exist. In fact, whenever a rare or comparatively unknown animal 

 or plant has been at any time supposed to be produced by sponta- 

 neous generation, it has only been necessary, for the most part, to 

 investigate thoroughly its habits and functions, to discover its secret 

 methods of propagation, and to show that they correspond, in all 

 essential particulars, with the ordinary laws of reproduction. The 

 limits, therefore, within which the doctrine of spontaneous genera- 

 tion can be applied, have been narrowed in precisely the same 

 degree that the study of natural history and comparative physiology 

 has advanced. At present, indeed, there remain but two classes 

 of phenomena which are ever supposed to lend any support to the 

 above doctrine ; viz., the existence and production, 1st, of infuso- 

 rial animalcules, and 2d, of animal and vegetable parasites. We 

 shall now proceed to examine these two parts of the subject in 

 succession. 



INFUSORIAL ANIMALCULES. If water, holding in solution or- 

 ganic substances, be exposed to the contact of the atmosphere at 

 ordinary temperatures, it is found after a short time to be filled 

 with swarms of minute living organisms, which are visible only by 

 the microscope. The forms of these microscopic animalcules are 

 exceedingly varied ; owing either to the great number of species 

 in existence, or to their rapid alteration during the successive pe- 

 34 



