444 Dr. V. Fatio on the Variahility of the 



These first two cases have often occurred to' me in the com- 

 parative investigation of the fishes in the Swiss lakes of the 

 north and south of the Alps. But there is a third case, with 

 regard to which I must here say a few words — namely, the 

 rare case in which we find suddenly, and as it were by chance, 

 among a great number of individuals of two derivations, and 

 sufficiently constantly different to appear to belong two species, 

 an individual which, in one of the geographically separated 

 forms, resembles the other form in all its characters, so as 

 actually to be mistaken for it, and thu.s betrays the heredity 

 or identity of origin. 



I may cite, as a curious example of this last case, the dis- 

 covery that I made in the lake of Lugano of a bleak [Albur- 

 nus) which, south of the Alps, perfectly recalls the form proper 

 to our representative of the genus north of that chain. It is 

 well known, in fact, that hitherto all ichthyologists have 

 recognized the bleak of the Ticino and of Italy as completely 

 and specifically distinct from that which inhabits the waters 

 which have their source north of the Alps. Now the speci- 

 men in question, found among hundreds of similar individuals 

 of Alhurnus alhorella^ presents, both in size and coloration, 

 and in its various forms and proportions, nearly all the cha- 

 racters regarded as distinctive of our Alburnus lucidus. Never 

 before has such a bleak been noticed as inhabiting the Italian 

 waters ; and it would be very difficult for me to explain under 

 what influence this reversion can have been produced. How- 

 ever, in presence of this revocation of consanguinity, I can now 

 do no otherwise than regard Alburnus lucidus and A. alborellaj 

 which at first sight are very distinct, as two races, the one 

 northern, the other southern, of one and the same species. 

 Although it would appear that we must go back very far to 

 find the epoch at Avhich these two supposed species lived under 

 the same form, under identical conditions, it none the less 

 seems that we have to do here with a complete case of 

 atavism^ though of very distant date. 



The partisans of the variability of species have laid much 

 stress on the study of the variations of domestic animals. 

 Deformations produced accidentally or by artificial selection 

 are, in fact, comparatively easy of demonstration in subjects 

 necessarily submitted to our observation ; but the appearance 

 in free creatures of modifications superinduced by natural se- 

 lection, or at any rate by influences independent of the will of 

 man, being always more difficult to seize, it seems that the 

 study, under natural conditions, of a divergence of any kind 

 in any organ must also possess its interest and value. 



Let us confine ourselves now to the study of the modifies- 



