( 427 ) 



varieties did not possess, besides the (morphological) distinguishing characters, either 

 antipathy against mutual intercrossing, or mutual sterility, or both qualities; and 

 he gives the following illustration : — 



" The caterpillars of the two closely allied species of butterflies Gonopteryx 

 rhamni and G. eleopatra became adapted to different species of plants, the one 

 to Rhainnus frangul.a [and cathartica], the other to Rhamnus alpina, i.e. the 

 apparatus of digestion became so modified that those species of plants could be made 

 use of [physiologically] in the best possible manner. With the condition of the organs 

 of digestion was associated, perhaps indirectly, a somewhat different yellow coloiu' in 

 the imago-state, and here the preference could begin to act. . . . Against a thousand 

 butterflies adapted to Rhamnus alpina there occurred perhaps two which were pre- 

 possessed in favour of a deeper yellow colour, two which jireferred a lighter yellow 

 tint, while those thousand were indifferent [to the deeper or lighter tint of vellow]. 

 The first-named [two] specimens copulated with individuals adapted to Rhamnus 

 alpinn, and gave birth only to ftilly fertile offspring. The second group [two speci- 

 mens] copulated with light-coloured individuals adapted to Rhamnus frangula, and 

 gave birth to non-fertile intergraduates. Of the thousand specimens one half copu- 

 lated with specimens adapted to Rhamnus alpina, the other half with specimens 

 adapted to Rhamnus franffula ; of the two halves, therefore, onlj'the first gave birth 

 to fully fertile offspring. ... It is easily to be seen that the number of those indi- 

 viduals which were prepossessed in favour of closely allied specimens [in colour] would 

 grow very quickly, and that the number of the others must very soon become reduced, 

 even if this number at the beginning [of the selection] is still more preponderating 

 [than in the above illustration]." 



As the author expressly states that the divarication of species at the same locality 

 is not possible if together with the " development of the distimjuishing characters 

 there did not take place a developjment of dislike," etc., we must take it as the meaning 

 of the illustration that Gonopteryx rhamni and G. cleopiatra have developed from 

 a common ancestor into two species b}- means of psychological selection. Or, to put 

 it figui'atively, the species A developed into two varieties, .^1' and A-, in consequence 

 of the caterpillars becoming changed by the influence of "the different food-plants; 

 then psychological selection on the part of some females set in and modified those 

 two varieties more and more, so that the varieties A^ and A^ became in the course of 

 time two " species" B and C. \\'Tiich characters does the author attribute to the two 

 varieties A' and A'- at the time when .selection commenced to act ? There were foiu' 

 distinguishing characters : (1) A' and A'- were different in the chemistry of their 

 body ; (2) they were so different in colour that the females could perceive the 

 difference ; (3) A^ produced only pale specimens, and A' only deeper-coloured speci- 

 mens, i.e. they bred true ; (4) the cross-products were not fit to propagate. Now, if 

 forms so widely distinguished moqihologically and physiologically as ^1' and A" are 

 not •' species," then we fear there are no species at all. The author apparently con- 

 founds the transmutation of one species into one other and the separation of a 

 species into two or more. The illustration does not show how by means of psycho- 

 logical selection (or any other modifying factor) G. deopjatra and G. rhavini have in 

 the same locality developed from one conmion ancestor into two species, but shows 

 how the two "species" eleopatra and rhamni might become the one a paler and 

 paler, the other a deeper and deeper yellow " species." At the time when selection 

 set in in Dahl's illustration there were already two " species," and hence the specific 

 distinctness of the two is not the outcome of psychological selection. 



