of the Fishery Board for Scotland. 



207 



return continually to similar spawning places under similar condition.-;, we 

 should be forced to admit the uselessness of searching for groups or 

 "races." It will be shown, however, on the contrary, that we have good 

 evidence to believe that through habit or instinct the animals of a group 

 return constantly to approximately the same places and conditions where 

 they were born in order to spawn. Thus, we have the essential 

 "inherited" conditions for the maintenance of a group or race. 



What would happen, however, if the conditions changed in the mean- 

 while ? Obviously we could not expect the "acquired characters" to 

 remain constant. If the change was so extreme that the variability 

 possessed by the organism could not respond to the impressed stimuli we 

 should expect that none of the offspring would survive. As an example 

 of this wholesale destruction of a group or species may be mentioned the 

 remarkable case of the tile-fish in American waters. If the variability 

 possessed by the organism was, however, equal to the emergency, then we 

 should expect that the new "acquired characters" would differ greatly 

 from the old, and would be similar to the characters of any other group 

 of the same species which had formerly lived under these new conditions. 

 If the new conditions had come to stay, might we not believe that 

 "through the nature of the organism and of surrounding conditions, but 

 not through natural selection,"* a new group or race had been formed, 

 and if a new group, why not a "new" species? 



This conceivable mode of the differentiation or formation of species, 

 is not to be confounded with "evolution" or the process by which 

 one form has developed from another so that the distinction of higher and 

 lower may be made — which is a related yet quite separate problem. 



The Various Forms op Variability. 



The questions in connection with variability resolve themselves into at 

 least three distinct main problems, which are so closely related to one 

 another that it is necessary to treat of them together, and yet so complex 

 and large that they demand separate attention and study. These 

 problems arise from the different ways that the variability of the organism 

 shows itself. There are (1) sex-variability ; (2) growth-variability', (3) 

 race- or family -variability. Another form of variability has been 

 suggested by Heincke, namely, asexual variability, which is meant to 

 include the differences that would be found between two groups of 

 animals after the other forms of variability have been eliminated. It is 

 doubtful, however, whether these differences would not come under the 

 heading (3) race-variability in the end. 



I. Sex-variability. — Sex-differences are divided into two classes — 

 primary, those directly concerned with the reproductive organs j 

 secondly, those not so connected, but in some way related to sexuality. 

 Only the latter group come into the present research. In the class of 

 fishes, as a whole, secondary sexual differences are not so well marked as 

 in other vertebrates, but where they do occur they manifest themselves in 

 a similar way, in colouration, structure of the jaw, for example. The 

 question is whether they occur in the plaice. 



ISTo very definite external differences have been found except in the 

 case of " ciliated scales," and even this is doubtful. Both Dunckert and 

 Cunninghami found that the mature males possessed these scales, but 



* Darwin, C. : "Origin of Species," ed. 6, p. 176; cited by Professor Sedgwick, I.e., 

 p. 12. 



t Duncker, G.: "Variation u. Verwandtsehaft v. P. fiesus L. u. P. platessa L.," Wisen 

 Meeresunter., B.T.H. 2, 1896. 



t Cunningham, J. C: "Peculiarities of Plaice from different Fishing Grounds," Jour. 

 Mar. Biol. Ass., Vol. TV., 1897, pp. 315-357. 



