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we may be in the dark as to the causes which produce 

 them. 



What indeed the exact limits may be, either in the ani- 

 mal or the vegetable kingdom, within which the power of 

 producing prolific offspring may be circumscribed, our pre- 

 sent knowledge is not sufficient to allow us to define ; but 

 this at least is certain, that both in the one and in the other 

 the deviations from the typical forms produced by this cause 

 are few and unimportant. On the hybridization of plants 

 indeed, Gsertner is said to have instituted no less than ten 

 thousand experiments, in the course of which he operated 

 upon seven hundred distinct species ; and yet he produced 

 only two hundred and fifty true hybrids, most of which 

 were feeble and barren; nor amongst the whole number 

 were there any that exhibited characters foreign from those 

 of their respective parents. 



Whence this difficulty in the way of producing hybrids 

 between allied forms should arise may perhaps ever con- 

 tinue a mystery; but the final cause of it seems obvious 

 enough, as species could not have been maintained in their 

 integrity, if intermediate forms were liable in this manner to 

 spring up, and to continue permanent. 



But, it may be asked, if the comparatively small variations 



