GENERAL CONSIDERATIONS. 1039 



those which we meet with between the different individuals of any one race. 

 Thus, we not only find the average duration of life to be the same (making 

 allowance for circumstances which are likely to induce disease), but the various 

 epochs of life 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 present a marked general uniformity, such as does 

 not exist among similar epochs in the lives of species that are nearly allied, but 

 yet unquestionably distinct. Further, the different races of Man are all subject 

 to the same diseases, both sporadic, endemic, and epidemic; the only exceptions 

 being those in which the constitution of a race has grown-to a certain set of 

 influences (as that of the Negro to the malaria which produces certain pernicious 

 fevers in the European), producing an hereditary immunity in the race, which 

 is capable of being acquired by individuals of other races, by a process of accli- 

 matization commenced sufficiently early. 1 The most important physiological 

 test, however, of specific unity or diversity is that furnished by the Generative 

 process. It may be considered as a fundamental fact, alike in the Vegetable 

 and in the Animal kingdom, that hybrid races, originating in the sexual con- 

 nection of individuals of two different species, do not tend to self-perpetuation; 

 the hybrids being nearly sterile with each other, although they may propagate 

 with either of their parent races, in which the hybrid race will soon merge ; 

 whilst, on the other hand, if the parents be themselves varieties of the same 

 species, the hybrid constitutes but another variety, and its powers of repro- 

 duction are rather increased than diminished, so that it may continue to pro- 

 pagate its own race, or may be used for the production of other varieties, almost 

 ad infinitum. It appears that, among Plants, hybrids originating between 

 undoubtedly distinct species, sometimes reproduce themselves for two or three 

 generations, but do not continue beyond the fourth. Amongst Animals., the 

 limits of hybridity between parents of distinct species are more narrow, since 

 the hybrid is totally unable to continue its race with one of its own kind; 3 and 

 although it may propagate with one of its parent-species, the progeny will of 

 course approach in character to the pure breed, and the race will speedily 

 merge into it. In Animals, as among Plants, the mixed offsprings, originating 

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

 vigor, and in the tendency to multiply, the parent-races from which they are 

 produced, so as to gain ground upon the older varieties, and gradually to super- 

 sede them. In this manner, by the crossing of the breeds of our domesticated 

 animals, many new and superior varieties have been produced. The general 

 principle is, then, that beings of distinct speciesj or descendants from stocks 



1 This view of the immunity of the Negro race from certain forms of Fever which are 

 very fatal to Europeans, is justified, the Author believes, by all the facts known upon the 

 subject. Much may be set down, as he is assured by Dr. Daniell, to the better adaptation 

 of the Negro habits of life to their climate ; and Europeans who exercise due caution 

 (especially in regard to the functions of the skin) may preserve an immunity scarcely less 

 complete. Dr. D. himself, having been taken prisoner by one of the Negro tribes at an 

 early age, and having spent two years among them, seems to have been thoroughly accli- 

 matized ; and has subsequently passed many years on the most unhealthy parts of the 

 coast, without experiencing any severe attacks of illness, and in the enjoyment of very 

 good general health. It is sometimes maintained that the Negro race possesses such a 

 complete exemption from the Yellow Fever of the United States, as marks its specific 

 difference ; such, however, is not constantly the case, since Negroes occasionally suffer 

 from it; and their comparative immunity seems fairly attributable to the constitutional 

 peculiarity acquired by their African progenitors, and capable of being acquired by Euro- 

 peans also. 



2 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 further than the second gene- 

 ration, without admixture with one of the parent-species. 



