11 



while and see what was to be made out of those already 

 accumulated. 



Believing that there were only the two main " races " — 

 the Manx and Welsh ones — we thought the best method was 

 to spread the collection of measurements over several years 

 and to " lump " together all the samples obtained from Isle of 

 Man, irrespective of the month or year of collection. So also 

 with the Welsh fish. In that way we hoped to get such big 

 series of data that the averages, and other statistical functions 

 deducible therefrom, would be fairly representative ones. 

 Now this method may be quite wrong. 



Even with quite small samples taken in the same region, 

 and after intervals of some weeks, there may be quite noticeable 

 differences : differences as big as those obtained when we 

 contrast the fish taken from Isle of Man with those taken from 

 the Welsh coasts. This may, conceivably, be the case even 

 if we hold that there are only the two main races. It may be 

 that in taking the sample this week we have " accidentally " 

 included more of the herrings that vary from the average in one 

 direction while, in the next small sample, we may have included 

 more of the herrings that vary from the average in the other 

 direction. If this is so there is a test, based by Professor Karl 

 Pearson on the statistical theory of random sampling, which 

 can be applied. 



But it is also possible, and some results of general biology 

 make it quite likely, that there is another explanation. It may 

 be that, instead of two main " races " there are really a number 

 of " sub-races," or " genotypes," that is, strains of herrings, 

 that are really permanent, or the same from generation to 

 generation, except in so far as they may vary by inter-mixture 

 with each other. This inter-mixture may, however, be regarded 

 as rather improbable because of the tendency of the herrings 

 to remain together as lonely aggregated schools. There are, 

 then, a number of strains, or genotypes, of Irish Sea herrings 



