16 John Frauds Walker — On Species. 



or from one of unusual size, or from a badly preserved specimen in 

 which the cliaracters are obscure. Many of the older species have 

 been made into subgenera, and these again divided into species. 

 The difference between a species and a variety is that the species is 

 more permanent, being in a state of stable equilibrium. 



I have stated (Davidson's " Suppt. Jurassic Brachiopoda," p. 180)- 

 that it is advisable to give a name to a well-marked variety when 

 it is constantly found in a given geological horizon, although it may 

 be connected by rare forms with other species, and also for the 

 purpose of limiting the extent of variation of a form bearing the 

 same name, so that there can be no doubt which variety is referred 

 to when it is stated to occur in a certain district. 



Species can be made to appear to have been joined together by 

 selecting the abnormal forms out of thousands of specimens and 

 neglecting the percentage of normal ones. The abnormal form of one 

 species may be the normal form of another species. The true centre of 

 a species may be found by collecting from a given locality a very 

 large number of specimens and observing the percentage of the 

 different forms. The greatest number of individuals closely re- 

 sembling each other will indicate the centre of the species at that 

 locality. It will be found that no two individuals are exactly alike, 

 but the amount of variation of those which form the bulk of the 

 collection is infinitesimal ; these are the centre of the species, whilst 

 the rarer forms differ from the typical specimens, and are therefore 

 further from the centre of the species. The conversion of one 

 species into another must bg gradual, as a sudden change would 

 probably make the species extinct, as it would require time to adapt 

 itself to the conditions which alter its form. Great and sudden 

 alterations in form tend to weaken an individual so that it seldom 

 survives ; this serves to check rapid variation. 



The tendency of variation in a species is also checked by their 

 interbreeding with each other, and probably the power of variation 

 during a limited time is not great without exhausting the species. 

 The susceptibility to the influence of environment will vary in 

 different individuals : some will be able to alter their form and adapt 

 themselves to the new conditions, others will scarcely be able to 

 alter and will become gradually extinct. 



The rate of valuation in individuals may be different, and give 

 rise to new species : thus, the species A may develop two or more 

 points nearer its circumference where individuals differing from 

 the typical form begin to thickly cluster, and so give rise to new 

 centres, the percentage of the normal form of the species A becoming 

 gradually less ; these centres, if isolated from each other, will give 

 rise to new species. The spaces between different centres of species 

 are unequal, owing to missing species which have not yet been 

 discovered. 



The duration of the life of a species will depend on the length 

 of time it exists under the same conditions ; the longer it does so 

 the less it will vary, as it will have gradually acquired the most 

 favourable form for its environment, and therefore all variation& 



