XVIII THE PROBLEM OF UTILITY 385 



Characteristics of Species due to Natural Selection. 



The sketch now given of the usual mode of formation 

 of new species under natural selection, leads to the. con- 

 clusion that every species (of the higher animals at all 

 events) will usually possess at least three peculiarities: in 

 the first place, it must exhibit some difference of structure 

 or function adapting it to new conditions ; secondly, some 

 distinction of colour, form, or peculiar ornament serving 

 as distinctive recognition marks ; and, thirdly, the physiolo- 

 gical peculiarity of some amount of infertility when crossed 

 with allied species. The first two constitute its " specific 

 characters." But if we consider that every species in the 

 long line of its ancestry must have had similar, but some- 

 what different specific characters, adapting it to the 

 peculiar conditions of its environment and distinguishing 

 it from its nearest allies ; that some of these characters, 

 when generally useful, have persisted, and now constitute 

 generic or family characters ; that others have been again 

 and again modified so as to adapt them to new and 

 sometimes quite different conditions ; and that others 

 again, becoming useless, persist when quite harmless or 



a number equal to the annual increase must die off every year, and 

 these will inevitably be, on the whole, those which are least 

 fitted to the new conditions, whether as J'oung or adult, and thus 

 natural selection begins to act in the production of a new form 

 suited to the new conditions. If the conditions are identical with 

 those of the former home of the species there will be no change ; 

 but if there are dififerences of conditions, either climatal, or in food 

 supply, or organic, as will in most cases be sure to exist, then there 

 will set in a gradual adaptation to these new conditions. Here arises 

 the circumstance that my critics have overlooked, that a modification 

 of one species into another must have occurred in every case, and 

 during that modification the need for some "recognition-marks" to 

 aid in checking the intercrossing of the incipient new species with 

 the parent form will be of the first importance. All variations in 

 colour or marking will be selected for this purpose, and thus will 

 be produced those external specific characters which are so marked 

 a characteristic of these insular forms. When the change has been 

 completed and the parent form has died out, these special and 

 characteristic markings or coloration will appear to us to be useless. 

 But they were essential to the development of the new species, and 

 having become fixed by long continued selection they now remain as 

 a constant specific character. 



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