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ON MECHANICAL SELECTION AND OTHEK PROBLEMS. 

 By KARL JORDAN, Dr. Phil. 

 [Plate.s XVI., XVII,, XVIII,, and XIX]. 



THE peculiar kind of variation I have to deal with in thi.< paper concerns .some 

 accessorv organs of the reproductive system of a group of insects, and bears 

 closely upon Komanes's theory of Physiological Selection. It is a jyrlorl evident 

 that the demonstration of the occurrence of an extensive variability in any part of 

 those organs which are the most important for the preservation of animated natm-e 

 must be of far-reaching consequence in respect to the origin of species. As the mere 

 statement of the facts would be unintelligible to the general reader who has no 

 special knowledge of the animals in question, I shall endeavour to interpret the facts. 

 However, before I begin to give the details, it is necessary to come to an understand- 

 ing about some questions of a general character, and this necessitates my entering, 

 rather reluctantly, upon a ground which nowadays has so often been traversed, with 

 and without success : I should not do so, if the facts I bring forth, and the con- 

 clusions I have to derive from them, did not put me in contraposition to a good 

 many naturalists. Being surrounded by cabinets full of specimens, I shall, during this 

 excursion on a theoretical ground, always be reminded that ])Ossibilities arrived at 

 by general reasoning are often impossibilities in natm-e, in so far as what is a jjviori 

 possible or even probable may be found not to occur in nature. 



I.— INTRODUCTORY NOTES. 



Everybody who is a little acquainted with the diagnostic works on Zoology or 

 Botany will know sufficiently that a continuous question of contest amongst us 

 species-makers is whether a given form of animal or plant is a " distinct species " or 

 not ; and he will also have become aware that in many cases the contending parties 

 do not come to an understanding, because, though using the same term " species," the 

 mutual conception of that term is widely different. And this is so not only amongst 

 us species-makers, but we meet with the phenomenon also in the es.^ays of a more 

 philosophical kind which bear upon the theor^y of evolution. It would almost appear, 

 in fact, as if a " species " is that which a respective author chooses to consider a 

 " species." It is not necessary to give any illustration taken from systematic works, 

 first, since we do not think it does much harm to the value of purely diagnostic articles 

 whether the term "species" is always applied in the .«ame sense by the same or 

 various authors ; .secondh', because illustrations can be found in any volume containing 

 descriptions of varieties and species. In natural philosophy, however, so far as it 

 endeavours to explain by theories the diversities in animated nature, it is all important 

 that an author has a fixed idea of that diversity wliich he in his writings calls 

 a "species," and therefore we will give here an illustration of the before-mentioned 

 l)henomenon taken from this side of our science. 



In a short article in which he claims priority over Romanes with regard to 

 physiological selection, Dahl * proceeds to say that a separation of tlie varieties of 

 one species into more species within the same locality is not possible, if (he various 



• Zool. Anzeigrr 1889. p. 262. 



