BASIS OF THE PKOBLEM 39 



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 tively as the sexual and asexual ; | the latter type of reproduction 



occurs only among plants and the lower animals. As sexual 

 reproduction occurs among all groups of animals and plants, 

 whether asexual reproduction occurs or not (except among the 

 bacteria where the latter type alone is known) and as it is the 

 only type which occurs among the higher groups, asexual reproduc- 

 tion may be disregarded here. The asexual type of reproduction, 

 it may be remarked, introduces no new principle, and those groups 

 among whom it is found are not on that account to be set apart 

 from other groups as though the general conclusions at which 

 we shall arrive were not applicable to them. 



To make the process of sexual reproduction clear it is necessary 

 to say something regarding the structure of organisms. Excluding 

 the Protista for the moment, all organisms exhibit a cellular 

 structure. Upon examination under a microscope the living 

 tissues are found to consist of a number of compartments or cells. 

 Among plants the typical cell is in the nature of a small box 

 with thick sides ; among animals the sides or cell-walls are very 

 thin. "Within the cell-walls there are to be distinguished two 

 chief elements, a central body, the nucleus, and the surrounding 

 cytoplasm. This cytoplasm consists of ordinary granular proto- 

 plasm and the nucleus of a special kind of protoplasm. Only 

 protoplasm can properly be spoken of as alive. Those elements 

 in the body which are not protoplasm are either in the course of 

 being turned into protoplasm or are the products of protoplasmic 

 activity. Examples of the latter in man are the hair and the 

 nails, and in the case of other animals, feathers, shells, and so 

 on. As a general rule, each cell contains a single nucleus, but 

 there are cells without a nucleus and cells with more than one 

 nucleus. The name Protista, mentioned above, is given to the 

 lowest animals and plants which are commonly said to consist 

 of a single cell. They may be thus thought of ; but the group 

 includes forms with many nuclei of which it can only be said that 

 cell structure has not yet been diiTerentiated. It is therefore 

 possible that the term cell should not be used at all in respect of 

 this group. 



Among the Protista sexual reproduction takes very different 

 forms. Essentially, however, it consists in the fusion of two 

 individuals, and if we regard each individual as a cell, it may 

 be said to consist in the fusion of two cells. Among the higher 



