670 NATURE OF REPRODUCTION. 



parents, although in all cases the final result of the generative process, 

 is never immediate. The young progeny when first produced is different 

 from its parents, and only reaches a condition of resemblance to them 

 through a series of changes, often of a very extraordinary kind. In 

 the vertebrate animals generally, the embryo, though quite incomplete 

 in structure, yet presents a certain analogy of form with the adult con- 

 dition. But in many of the invertebrate animals the young, even after 

 hatching, and when capable of active locomotion, are so different in 

 appearance from their parents that they would never be supposed to 

 belong to the same species, unless their identity were demonstrated by 

 their subsequent development. Thus the young mosquito is a wingless 

 creature living beneath the surface of the water in stagnant pools j and 

 the eggs of the butterfly, when hatched, give birth not to butterflies but 

 to caterpillars. These caterpillars, however, are not creatures of a 

 different species, but only young butterflies ; and they become fully 

 developed and similar to their parents after certain changes, which take 

 place at definite periods of their development. 



The reproduction or repetition, therefore, of the form which distin- 

 guishes a particular species is accomplished by a series of changes which 

 follow each other in regular order; and this series, taken together, may 

 be represented by a circuit, which starts from the egg, is continued 

 through the different phases of growth, transformation and maturity of 

 the animal, and terminates again with the production of an egg. As 

 this egg is similar to the first, the changes repeat themselves in their 

 previous order, and the indefinite continuance of the species is thus 

 established. 



Spontaneous Generation The commonest observation shows that 



the facts detailed above hold good in regard to all animals and plants 

 with whose history we are familiarly acquainted. An opinion, however, 

 has sometimes been entertained that there may be exceptions to this 

 rule; and that living beings can, under certain circumstances, be pro- 

 duced from inanimate materials ; presenting, accordingly, the singular 

 phenomenon of a progeny without parents. Such a production of 

 organized bodies is known by the name of spontaneous generation Its 

 existence is doubted by most physiologists at the present time, and has 

 never been positively established for any particular organized species; 

 but it has been at various periods the subject of active discussion, 

 forming a somewhat remarkable chapter in the history of general 

 physiology. 



It may be remarked in general terms that the organisms, in regard 

 to which the idea of the possibility of spontaneous generation has been 

 entertained, have been always those whose natural history was imper- 

 fect or obscure, owing either to their minute size or to certain of their 

 physiological peculiarities. Wherever animals or plants appeared in 

 considerable abundance without exhibiting any evidence of the source 

 from which they came, it was formerly conjectured, from that fact alone, 

 that their production was a spontaneous one. The ancient naturalists 



