82 MUTUAL RELATIONS OF THE HUMAN FAMILY. 



rnena presented by the domesticated species among the lower animals, is de- 

 cidedly in favour of the specific unity of the Human race; the differences 

 which have sprung up, in course of time, amongst the inhabitants of different 

 parts of the world, being such as we have a fair right to attribute according to 

 the recognized principles of Zoology to the modifying influence of external 

 conditions, acting upon a constitution peculiarly disposed to yield to it. 



5. On the Value of Physiological and Psychological Peculiarities, as 



Specific Distinctions. 



74. We have now to inquire, in the fourth place, what other arguments in 

 favour of this position may be drawn from agreement or difference in Physi- 

 ological and Psychological peculiarities. A comparison of the physiological 

 history of two races, is often found to afford a better criterion of their specific 

 difference or identity, than the comparison of their structural characters. 

 Now, in every important point of physiological history, there is a wonderful 

 agreement amongst the different races of Men ; the variations not being greater 

 than are those with which we meet among the different individuals of any 

 one race. Thus, we not only find the average duration of life to be every- 

 where the same, (making allowance for circumstances which are likely to in- 

 duce disease), but the various epochs of life have a close correspondence, 

 such as the times of the first and second dentition, the period of puberty, the 

 duration of pregnancy, the intervals of the catamenia, and the time of their 

 final cessation. And the different races of Man are all subject to the same 

 diseases, both sporadic, contagious, and epidemic ; whilst there are no two 

 really-distinct species among the lower animals, which have more than a very 

 slight conformity in this respect. 



75. The most important physiological test of specific unity or diversity, is 

 derived from the phenomena attending the Reproductive process. It is well 

 known that, in Plants, the stigma of the flower of one species may be fertil- 

 ized with the pollen of an allied species ; and that, from the seeds produced, 

 plants of an intermediate character may be raised. These hybrid plants, 

 however, will not perpetuate the new race ; for, although they may ripen their 

 seed for one or two generations, they will not continue to reproduce them- 

 selves beyond the third or fourth. But, if the intervention of one of the pa- 

 rent species be employed, its stigma being fertilized by the pollen of the 

 hybrid, or vice versa, a mixed race may be kept up for some time longer ; 

 but it will then have a manifest tendency to return to the 'form of the parent 

 whose intervention has been employed. Where, on the other hand, the pa- 

 rents themselves were only varieties, the hybrid forms but another variety, 

 and its powers of reproduction are rather increased than diminished ; so that 

 it may continue to propogate its own race, or may be used for the production 

 of other varieties, almost ad infinitum. In this way, many beautiful new 

 varieties of garden flowers have been obtained ; especially among such species 

 as have a natural tendency to change their aspect. Amongst Animals, the 

 limits of hybridity are much more narrow, since the hybrid is totally unable 

 to continue its race with one of its own kind ;* and although it may be fertile 

 with one of its parent species, the progeny will of course approach in cha- 

 racter to the pure breed, and the race will ultimately merge into it. On the 

 other hand, in Animals, as among Plants, the mixed offsprings originating from 

 different races within the limits of the same species, generally exceed in vi- 



* One or two instances have been stated to occur, in which a Mule has produced offspring 

 from union with a similar animal; but this is certainly the extreme limit, since no one has 

 ever maintained that the race can be continued farther than the second generation, without 

 admixture with one of the parent species. 



