Species in the case of certain Fishes. 443 



ever, it none the less appears, from what they say, that to them, 

 as to me and many others, variability is becoming more and 

 more evident. 



A variety duly ascertained may be regarded, according to 

 the point of view that we take up, either as a bond between two 

 so-called recognized species, or as a tendency towards the crea- 

 tion of a new form. 



The question as to the existence of a limit to the variability 

 of the species appears difficult to settle ; nevertheless it may 

 be remarked that, for the perpetuation and constant augmen- 

 tation of a deviation at one point, the gradual establishment 

 of a certain necessary equilibrium in the correlative variations 

 is required. A rapid modification of an organ Avliich, in con- 

 sequence of internal incompatibilities or external contrarie- 

 ties, is not followed quickly enough by corresponding changes 

 in other parts of the organism, will almost always superin- 

 duce either an arrest of the transformation in this direction, 

 or the extinction of the new divergent form, whether we regard 

 the latter as a species, as a variety, or simply as an abortive 

 shoot on a genealogical branch. 



A great number of observations tend to prove more and 

 more that, in the struggle for existence, natural selection 

 always gives the victory to the best organized, and that the 

 case of the strongest is always the best. Influenced in vari- 

 ous directions a species will give origin to several more or less 

 different offshoots ; and only those will long persist which may 

 be sufficiently strong to bend, tvithout excess and in an equi- 

 librated manner, to the various exigencies of different condi- 

 tions. 



There is therefore a limit, in a certain sense ; but this limit, 

 being due to a rupture of equilibrium, and often accidental, is 

 wider or narrower for the different varieties ; and each of the 

 latter, by departing more and more fi'om the type, always 

 runs the risk of meeting reverses in some part or other of its 

 organization lohen in a false direction. 



A natural barrier, even when very narrow, sometimes 

 suffices to establish differences, striking enough at the first 

 glance, between two allied forms. If, in the examination of 

 a great number of individuals taken imder the two conditions, 

 we can still perceive the transitional steps which explain the 

 series of transformations, we must, I think, for the time, 

 regard these two still divergent or parallel forms only as 

 local races of the same species ; but, on the contrary, if one or 

 several important steps are wanting in the scale of compari- 

 sons, we may regard these two opposed forms as different 

 species, until evidence to the contrary is produced. 



