2 SEX AND HEREDITY 



strands that mature into wood and bast in older trunks. 

 Such tissue-masses are familiar to everyone. Their micro- 

 scopic study shows that, whether in Animals or Plants, 

 these tissues are all made up of certain structural units, 

 called cells. Each cell is so minute that as a rule it 

 cannot be seen with the naked eye. It consists essen- 

 tially of a viscous body of protoplasm, a substance of the 

 nature of white-of-egg, in which all the activities of Life 

 are carried out. Within it lies a definite body of roundish 

 or oval form, the nucleus, which is itself part of the Proto- 

 plasm. It appears to dominate the cell, and serves as 

 the centre of its vitality. Very simple and minute 

 organisms may consist of only one cell each. These are 

 called unicellular organisms, and they lie at the base of 

 the series either of Animals or Plants. But those Animals 

 or Plants which are ordinarily known as such by the 

 general public are multicellular . They are composed of 

 very many, even of millions of these structural units, or 

 cells, which live together a common life, and compose 

 what is known as an individual, such as a horse or a tree. 

 We may go further and say that the whole of each Animal 

 or Plant is composed of such cells, or their derivatives. This 

 has been called the cell-theory. But it is now so fully 

 proved in detail that it may be stated not as a theory but 

 as a fact of observation (Fig. i). 



There is reason to believe that Animals and Plants have 

 advanced independently in the origin of Sex. But not- 

 withstanding this, one of the most striking features in 

 the whole story is that the leading facts of sexuality in 

 Plants and Animals are very much alike. Their similarity 

 is indeed so great that the same terms may be used in 

 describing both. The essential feature of Sexuality 

 whether in Animals or Plants, consists in the fusion of two 



