Mr. J. D. Dana on Species. 489 



despair in the thoughtful mind, instead of supplying its aspira- 

 tions with eternal and ever-expanding truth. It would be to 

 man the temple of nature fused over its whole surface and 

 through its structure, without a line the mind could measure or 

 comprehend. 



Looking to facts in nature, we sec accordingly everywhere, 

 that the purity of species has been guarded with great precision. 

 It strikes us naturally with wonder, that even in senseless plants, 

 without the emotional repugnance of instinct, and with repro- 

 ductive organs that are all outside, the free winds being often 

 the means of ti'ansmission, tliere should be rigid law sustained 

 against intermixture. The supposed cases of perpetuated fertile 

 hybridity are so exceedingly few as almost to condemn them- 

 selves, as no true examples of an abnormity so abhorrent to the 

 system. They violate a principle so essential to the integrity of 

 the plant-kingdom, and so opposed to nature's whole plan, that 

 we rightly demand long and careful study before admitting the 

 exceptions. 



A few words will explain what is meant by perpetuated fertile 

 hybridity. The following arc the supposable 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 connexion with the pure 

 or original stock. 



3. Mules that are wholly infertile among themselves, but 

 may have issue for a genei'ation or two by connexion with one 

 of the original stock. 



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

 have issue through indefinite generations by connexion 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 many 

 generations. 



7. 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 seve- 

 rally less and less numerous, and art must generally use an un- 

 natural 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. 



Ann. Sf Mag. N. Hist. Ser. 2. Vol. xx. Suppl. 32 



