AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 81 



8. 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 indefinite 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 permanence. The examples under the 

 heads 2 to 5 become severally less and less numerous, and art must generally use 

 an unnatural play of forces or arrangements to bring them about. 



Again, in the animal kingdom, there is the same aversion in nature to inter- 

 mixture, 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 system. It is treated like a disease, and the 

 energies of the species combine to throw it off. The short run of hybridity be- 

 tween the horse and the ass, species very closely related, reaching its end in one 

 single generation, instead of favoring the idea that perpetuated 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 necessity of finiteness 

 and fixedness for the very existence of a kingdom of life, or of human science its 

 impress on finite mind, are hence strong arguments for the belief that hybridity 

 cannot seriously trifle with the true units of nature, and at the best can only malie 

 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 allowed ; and that a closer and a 

 closer affinity might give a longer and a longer range of fertility. But the case 

 just now alluded to seems to cut the hypothesis short ; and moreover it is uot 

 reasonable to attiibute such indefiniteness to nature's outlines, for it is at variance 

 with the spirit of her system. 



Were such a ease demonstrated by well established facts, it would necessarily be 

 admitted ; and I would add, that investigations directed to this point are the most 

 important that modern science can undertake. But until proved by arguments 

 better than those drawn from domesticated animals, we may plead the general 

 principle against the possibilities on the other side. If there is a law to be dis- 

 covered, it is a wide and comprehensive law, for such are all nature's principles. 

 If ature will teach it not in one corner of her system only, but more or less in every 

 part. "We have therefore a right to ask for well defined facts, taken from the 

 study of successive generations of the interbreeding of species known to be distinct. 



Least of all should we expect that a law, which is so rigid among plants and the 

 lower animals, should have its main exceptions in the highest class of the animal 

 kingdom, and its most extravagant violations in the genus Homo ; for if there are 

 more than one species of Man, they have become in the main indefinite by inter- 

 mixture. The very crown of the kingdom has been despoiled ; for a kingdom in 

 nature is perfect only as it retains all its original parts in their full symmetry, uude- 

 VOL. III. F 



