Organic Evolution. 231 



breeds ancestral traits lie latent. The crossing of distinct 

 varieties may either neutralize the variations artificially 

 selected, and thus allow the ancestral characters which 

 have been masked by them to reappear ; or they may 

 allow the elements of the ancestral traits, long held apart 

 in separate breeds by domestication, to recombine with the 

 consequent emergence of the normal characters of the wild 

 species. But, in truth, any attempted explanations of the 

 facts are little better than guess-work. There are the 

 facts. And the importance of crossing as a determining 

 condition in domesticated animals should make us cautious 

 in applying reversion, as it occurs in such cases, to wild 

 species which live under more stable conditions where 

 crossing is of rare occurrence. 



The Origin of Variations. 



The subject of the origin of variations is a difficult one, 

 one concerning which comparatively little is known, and 

 one on which I am not able to throw much light. 



Taking a simple animal cell as our starting-point, we 

 have already seen that it performs, in primitive fashion, 

 certain elementary and essential protoplasmic activities, 

 and gives rise to certain products of cell-life. In the 

 metazoa, which are co-ordinated aggregates of animal 

 cells, together with some of their products, there is seen a 

 division of labour and a differentiation of structure among 

 the cells. We see, then, that variation among these related 

 cells has led to differences in size, in form, in transparency, 

 and in function ; while the cell-products have been differ- 

 entiated into those which are of lifelong value, such as 

 bone, cartilage, connective tissue,, horn, chitin, etc., 

 together with a variety of colouring matters ; those which 

 are of temporary value, such as the digestive secretions, 

 fat, etc. ; and those which are valueless or noxious, such as 

 carbonic acid gas and urea, which are excreted as soon as 

 possible. Here are already a number of important and 

 fundamental variations to be accounted for. 



