528 NATURE OF REPRODUCTION. 



derived, by direct reproduction, from previously existing parents 

 of the same species. As this, however, is a question of some im- 

 portance, and one which has been frequently discussed in works on 

 physiology, we shall proceed to pass in review the facts which have 

 been adduced in favor of the occurrence of spontaneous generation, 

 as well as those which would lead to its disproval and rejection. 



It is evident, in the first place, that many apparent instances of 

 spontaneous generation are found to be of a very different character 

 as soon as they are subjected to a critical examination. Thus grass- 

 hoppers and beetles, earthworms and crayfish/ the swarms of minute 

 insects that fill the air over the surface of stagnant pools, and even 

 frogs, moles, and lizards, have been supposed in former times to be 

 generated directly from the earth or the atmosphere ; and it was 

 only by investigating carefully the natural history of these animals 

 that they were ascertained to be produced in the ordinary manner 

 by generation from parents, and were found to continue the repro- 

 duction of their species in the same way. A still more striking 

 instance is furnished by the production of maggots in putrefying 

 meat, vegetables, flour paste, fermenting dung, &c. If a piece of 

 meat be exposed, for example, and allowed to undergo the process 

 of putrefaction, at the end of a few days it will be found to contain 

 a multitude of living maggots which feed upon the decomposing 

 flesh. Now these maggots are always produced under the same 

 conditions of warmth, moisture and exposure, and at the same stage 

 of the putrefactive process. They are never to be found in fresh 

 meat, nor, in fact, in any other situation than the one just mentioned. 

 They appear, consequently, without any similar individuals having 

 existed in the same locality ; and considering the regularity of their 

 appearance under the given conditions, and their absence elsewhere, 

 it has been believed that they were spontaneously generated, under 

 the influence of warmth, moisture, and the atmosphere, from the 

 decaying organic substances. 



A little examination, however, discovers a very simple solution 

 of the foregoing difficulty. On watching the exposed animal or 

 vegetable substances during the earlier periods of their decompo- 

 sition, it is found that certain species of flies, attracted by the odor 

 of the decaying material, hover round it and deposit their eggs 

 upon its surface or in its interior. These eggs, hatched by the 

 warmth to which they are exposed, produce the maggots ; which 

 are simply the young of the winged insects, and which after a time 

 become transformed, by the natural progress of development, into 



