98 SECTIONAL ADDRESSES 



sexual organs — certain subspecies of Ectoparasites ' are even mainly based 

 on differences in the ducts of the sexual organs and their accessory glands. 

 Systematics and morphology are different expressions for the same kind 

 of research, and I have no doubt that experimental biology will likewise 

 have such a deepening influence on systematics that the superficial gap 

 existing between the two lines of research will disappear too. Knowledge 

 begins with the observation of phenomena, not with the experiment. 

 The areas inhabited by the geographical forms of the species we have 

 studied are either strictly separated, as in the case of island forms, or they 

 are contiguous, there being between the areas no gap uninhabitable for 

 the species, such as water would be for a dryland species, or a desert 

 or savannah for a woodland species ; or the areas may overlap. What 

 happens when the areas touch or overlap and the geographical forms 

 come in contact with one another ? In a critical survey of the birds of 

 Kenya Colony, lately published by Dr. van Someren in the Tring Museum 

 periodical,® every now and again the author records the observation that 

 perfectly distinguishable subspecies intergrade in the intermediate district, 

 where the two evidently have interbred and produced an impure popu- 

 lation, not strictly distinguishable from, nor identical with, either present 

 subspecies. The phenomenon occurs very frequently, as must be ex- 

 pected ; for the breaking-up of a species into geographical units cannot 

 at once result in sexual aloofness. This, however, is a point which should 

 be further investigated. Standfuss ^ mentions, for instance, that, accord- 

 ing to his experience, specimens from different districts do not mate so 

 easily when brought together as do specimens from the same district. It is 

 therefore quite possible that geographically separate populations which 

 the systematist considers identical, because he does not find any morpho- 

 logical differences, may nevertheless have acquired a physiological differ- 

 ence, the rudiments of a physiological barrier, which the experiment only 

 could detect. Though hybrid populations are of common occurrence, 

 they have not been thoroughly tested with some exceptions. Prof. W. F. 

 Balfour-Browne made the interesting discovery among the water-beetles 

 of Great Britain — ^and Mr. J. O. Cooper has corroborated the discovery 

 in other species — that there is a certain species in the south of Great 

 Britain and another in the north, clearly differentiated, while in the inter- 

 mediate area both are found with all intergradations. The species-pairs, 

 as Prof. Balfour-Browne calls them, are of great significance. The 

 systematist does not know what to do with them ; he generally treats the 

 hybrid population as an intermediate race and gives it a name or leaves 

 it without one of its own ; the insect catalogues abound with the name 

 ' intermedia.' I need hardly point out that the frequency of the occurrence 

 of mixed blood is a snare for the geneticist who bases his conclusions on 

 the assumption that the original specimens for his series of experiments 

 were pure, while his experimental results may in reality be due to 

 the hybrid nature of the parent stock. Intermediate races fluctuating 

 in character are often indefinable morphologically, but are definable 



' Cf. Jordan, Trans. Fourth Intern. Congr. Entom., p. 498 (1929). 

 ^ Nov. Zool., vol. xxxvii, p. 292 (1932). 

 * Handbuch, p. 107 (1896). 



