374 Thoughts on Sjpecies. 



A few words will explain what is meant by perpetuated fertile 

 tybridity. The following are the supposeable grades of results 

 from intermixture between two species : — 



1. No issue whatever — the usual case in nature. 



2. Mules (naming thus the issue) that are wholly infertile 

 whether among themselves or in case of connection with the 

 pure or original stock. 



3. Mules that are wholly infertile among themselves, but may 

 have issue for a generation or two by connection with one of the 

 original stock. 



4. Mules that are wholly infertile among themselves, but may 

 have issue through indefinite generations by connection for each 

 with an individual of the original stock. 



5. Mules that are fertile among themselves through one or 

 two generations. 



6. Mules that are fertile among themselves through an indefi- 

 nite number of generations, 



The cases 1 to 5 are known to be established facts in nature ; 

 and each bears its testimony to the grand law of purity and per- 

 manence. The examples under the heads 2 to 5 become sever- 

 ally less and less numerous, and art must generally use an unna- 

 tural play of forces or arrangements to bring them about. 



, Again, in the animal kingdom, there is the same aversion in 

 nature to intermixture, and it is emotional as well as physical. The 

 supposed cases of fertile hybridity are fewer than among plants. 



Moreover, in both kingdoms, if hybridity be begun, nature 

 commences at once to purify herself as of an ulcer on the sys- 

 tem. It is treated like a disease, and the energies of the spe- 

 cies combine to throw it off. The short run of hybridity between 

 the horse and the ass, species very closely related, reaching its 

 end in one single generation, instead of favoring the idea that per- 

 petuated fertile hybridity is possible, is a speaking protest against 

 a principle that would ruin the system if allowed free scope. 



The finiteness of nature in all her proportions, and in the ne- 

 cessity of finiteness and fixedness for the very existence of a king- 

 dom of life, or of human science its impress on finite mind, are 

 hence strong arguments for the belief that hybridity cannot se- 

 riously trifle with the true units of nature and at the best can 

 only make temporary variations. 



It is fair to make the supposition that in case of a very close 

 proximity of species, there might be a degree of fertile hybridity 



