582 REPRODUCTION. 



often in direct proportion to the length of the boiling process. Further- 

 more, in certain cases the bacteria failed to be produced at all, and the 

 chance of their production was found to decrease in proportion to the 

 time during which the liquid had been boiled. Thus, of four series of 

 flasks, containing the same infusion, and boiled respectively for one, two, 

 three, and four hours, all of the first and second series produced bacteria, 

 only one of the third, and none of the fourth. Finally, in no instance, 

 among numerous trials, did they appear in any infusion which had been 

 boiled for a period exceeding five hours. Thus a limit was reached to 

 the production of bacteria, in fluids previously subjected to the action 

 of heat. 



There can be no doubt as to the bearing of these and similar experi- 

 ments. Spontaneous generation is inadmissible at the present day for 

 everything except bacteria ; and with regard to them there is no suffi- 

 cient proof of their production independently of previously existing 

 germs. 



Sexual Generation. 



In all the higher plants and animals generation is accomplished by 

 the union of the sexes. Each sex is distinguished by special genera- 

 tive organs, male or female, which give rise to a peculiar organized 

 product; and this, product unites with that from the opposite sex, to 

 form a new individual. The female organs produce an egg or germ 

 capable of being developed into the young animal or plant ; the male 

 organs produce the sperm or spermatic fluid, necessary to fecundate 

 the germ and communicate to it the stimulus of development. 



In flowering plants, the female product is the " germ ;" which, after 

 fecundation by the male product or " pollen," becomes the seed or fruit, 

 and may produce a new plant by further development. In many spe- 

 cies, as in the lily, the violet, the convolvulus, both male and female 

 organs are contained in the same flower ; in some there are separate 

 male and female flowers on the same plant, as in the oak, beech, birch, 

 and hickory ; and in others, as in the willow, poplar, and sassafras, the 

 male and female flowers are on different plants of the same species. 



In animals the female organs produce the "ovum" or egg, and are 

 called ovaries. The male organs, which give rise to the spermatic 

 fluid, are the testicles. In some invertebrate species, as in the snail, 

 slug, leech, and earthworm, both ovaries and testicles are present in 

 the same individual. But impregnation is nevertheless effected by the 

 sexual union of two organisms ; the eggs produced by one animal being 

 fecundated by the seminal fluid of another, and vice versa. 



In all vertebrate animals, the two sets of generadve organs are 

 located in separate individuals; and the species is divided into two 

 sexes, male and female. There are also, for the Taost part, accessory 

 organs of generation, which assist in the accomplishment of the pro- 

 cess, and occasion a corresponding difference in the bodily form. In 

 some cases this difference is so great that the male and female would 

 never be recognized as belonging to the same species, unless they were 



