150 LECTURES ON BIOLOGY 



strating its presence. Whilst we are able to observe at the first 

 glance the individual characteristics of those belonging to our own 

 race, and even recognize after a lapse of years persons whom we 

 met perhaps only once before, all persons who go to a country 

 peopled with a different race find it very difficult to distinguish 

 between the natives. At first it seems to them as if all people look 

 exactly alike, at any rate those of the same age and sex, or those 

 that are not accidentally marked by some distinguishing feature. 

 Only long habit and practice enable the eye to discern the 

 personal distinctions of these foreign races, and w r e become 

 gradually accustomed to discriminate as easily between those 

 individuals as between our own countrymen. Still more difficult 

 it is of course to discern individual distinctions in animals which 

 are so much more differentiated from us in their organization. 

 Here the individual disappears, as it were, entirely behind the 

 type, and only a trained eye is able to perceive individual varia- 

 tions, especially in the lower animals. If, for instance, we 

 observe with an untrained eye a large flock of sheep we shall 

 only be able, at the best, to distinguish the bucks, the ewes, and 

 the lambs, but all the other sheep would seem exactly alike, 

 But a good shepherd knows each single individual of his flock, 

 though it may often number thousands. The large sheep- 

 breeders of Australia possess this skill to a remarkable degree, 

 and the rapidity and certainty with which they pick out a 

 certain animal, though it does not appear to be distinguished by 

 any special mark, is almost incredible. In the same manner 

 many gamekeepers are able to recognize every head of game in 

 their preserves. 



Thanks to the careful investigations and laborious measure- 

 ments which are now undertaken on our biological stations 

 we possess conclusive proof that even important organs may be 

 subject to great modifications in different individuals living 

 under identical conditions. Professor Heincke, of the biological 

 station in Heligoland, made such investigations with the herring 

 and was able to demonstrate a most extraordinary deviation in 

 the structure of fishes inhabiting the same district. To men- 

 tion only one instance, he showed that the number of vertebrae 

 fluctuated in different fishes between fifty-three and fifty-eight. 

 That certain parts in man are subject to similar fluctuations we 



