SURVIVAL OR DISAPPEARANCE OF NEW FORMS 399 



of changes in the protoplasm due to internal causes 

 which we cannot yet trace. 



Another and probably very important cause of the 

 appearance of new species is the crossing of distinct 

 but closely allied species, the gametes contributing to 

 the zygote protoplasm (chromatin) of slightly different 

 chemical composition or physico-chemical structure, and 

 thus producing an individual of a new type. 



Not all new forms which come into existence in one 

 of these ways succeed in surviving and maintaining 

 themselves. Some of the changes that may occur 

 may make the protoplasmic ''machine" unworkable, and 

 in that case the new organism dies and no new species 

 appears. In other cases the change may lead to a 

 modification of structure or function which is badly 

 out of harmony with the conditions in which the new 

 organism finds itself. For instance, to take a purely 

 hypothetical case, a seed plant living in a climate where 

 it was occasionally exposed to very dry air might 

 undergo a protoplasmic change which led to the cells 

 of the epidermis being unable to form a good cuticle, 

 so that the plant would rapidly lose water by evapora- 

 tion from its epidermal cells. Such a plant would dry 

 up and die, and the new form would never establish 

 itself. 



But in some cases the change may actually lead to 

 the new form being better adapted to the life conditions, 

 and in that case the new form will not only survive, 

 but will have an advantage in the struggle for existence. 

 We saw in the last chapter that a very small proportion 

 of the seeds produced by a seed plant actually find 

 suitable conditions for germination, and that a very 

 small proportion of the seedlings produced by the 

 seeds which do germinate grow into adult plants which 



