of the Fishery Board for Scotland. 



221 



When these two forms of variability have been eliminated a multi- 

 maximal curve may still arise from two causes — variability in time or age 

 and variability in space. Both of these are included under the term race- 

 variability, and it has already been shown that the difference between 

 them and the latter is in each case only a matter of degree. Age or time- 

 variability arises from the prolonged spawning period of a fish. An 

 individual born at the beginning of this period cannot be exactly the same 

 as an individual born at the end of the period. This is probably the chief 

 cause of the range of variations in a character. If the spawning period 

 has only one maximum, as is believed to be the case for each group of 

 plaice, then it is improbable that a multimaximal curve of variations will 

 arise from this cause. In the case of the herring, however, Heincke has 

 shown that a large group of individuals inhabiting the same, or nearly the 

 same, region may be divided up into two separate groups on account of 

 this age-variability. Although connected by intermediate stages, there 

 are, for example, two maximal spawning- periods for the herring of the 

 east coast of Scotland, and in correspondence with this it is found that 

 the averages of the variations in certain characters of these two groups 

 are distinct from one another, although the ranges of variations overlap. 

 Hence, we see how age-variability grades into race-variability and how a 

 unimaximal may become a multimaximal curve. The degree at which 

 the one passes into the other can be determined from the inspection of 

 characters only by the method of probability already illustrated. 



Again, multimaximal curves of variation and race-distinctions may arise 

 from differences in geographical distribution, or space-variability. Indi- 

 viduals born at the same time but in different regions will differ more in 

 their characters from those born at the same time and in the same region, 

 and the wider apart the regions are the more likely are they to give rise 

 to race-differences. The latter, therefore, arise from the former by 

 differences of degree only. 



It is this latter form of variability that has been found in the plaice, so 

 far as observation has gone. It should be remembered, however, that 

 race-variability does not necessarily arise from the time and space 

 differences. Accompanying these must be other factors whose exact 

 importance in any one case is not yet known. It is not difficult to see 

 that the environmental conditions must differ just as much as the 

 characters before race-differences can be said to be present, but this is not 

 all. The characters of one generation in one local form may differ from 

 those of the same generation in another local form ; yet if these differences 

 do not remain from generation to generation we are hardly justified in 

 talking about "races," 



Avoiding the question of heredity, this problem may be stated in 

 another way. We may believe, in opposition to Bateson (I.e.), that the 

 environmental conditions may be discontinuous as the " species " may be 

 continuous — in the sense that he uses these words — because the environ- 

 mental conditions of the whole life-cycle of the species must be considered, 

 and yet the question will still arise, how do the " species " or " races " 

 remain distinct and separate? Are the races " isolated " from one another 

 either in time or space or physiologically by definite barriers 1 Most 

 biologists who have written on the subject of isolation within recent 

 years have considered that some such barriers were necessary, and Darwin 

 himself, in the later editions of the " Origin of Species," after the effects 

 of inter-crossing were pointed out to him, was inclined to agree with these 

 views. Yet his original conception seems most suited to what we find 

 amongst fishes at any rate. On a widely extended area, such as that 

 occupied by the herring or plaice as species, for example, there need be no 

 definite geographical or other barriers, and yet we can conceive how 



