LARIDM. 59 



The mouth of the Eden, on the east coast of Scotland, is a great breedmg-place for 

 Terns. The Common Tern there appears to lay three distinct varieties of eggs. A very 

 old St. Andrews naturalist first pointed this fact out to me. He showed me some hundreds 

 of Common Terns' eggs that he himself had collected, which he had divided into three 

 groups, calling them the "grass," the "heather," and the "sand." Sure enough the 

 "grass" egg exactly resembled the striped shadows grass would make, the "heather" was 

 spotted and speckled, and the "sand" was one uniform dark muddy sand-colour. Each 

 egg, in fact, possessed a remarkable resemblance in colour to its surroundings. Is it then 

 that the bird, after choosing a position for its nest, has the power of laying eggs of a 

 colour corresponding to the surroundings ? I think not. Is it then that the eggs of 

 themselves take up this colour ? Certainly not, or all eggs would be similarly affected, 

 and this we know is not the case. No ; it rather appears to be another example of 

 Darwin's " survival of the fittest." Taking into consideration the now well-known fact 

 that the dispositions and actions of animals are hereditary, a long ancestral line of birds 

 would year after year build in the same sort of locality, and those eggs resembling that 

 locality would be the most likely to escape destruction from their numerous enemies, and 

 so eventually establish three varieties of the Common Tern laying eggs like their surroundings. 

 One would therefore be inclined to say that such a disposition in time must constitute sufficient 

 specific distinctness in the bird itself to form three separate species. Darwin tells us that 

 the Tiger and Leopard are branches from the same stock. Two varieties were spared : those 

 most resembling the stripes like the shadows of the tall jungle-grass, and those most 

 resembling the spotted appearance of the shadows caused by leaves — ^eventually forming 

 the two present distinct species, though at what precise moment a variety turns into a 

 species is not quite clear.* 



While on the subject of Terns' eggs, I may mention a story a very keen old collector 

 once told me. He had found some Terns' eggs very much " sat on," and taking them 

 home he hatched them oif in hot water. He then put the young birds in a basket by 

 the fire, when, to his horror, a cat immediately took possession of the brood — not, 

 however, to eat, but to rear, which she did so successfully that they all grew up and 

 finally flew away out of the window. This sounds like a fairy tale, but such was the 

 story, and he seemed a truthful old man. 



The bird is thus described by Yarrell (Brit. Birds, vol. iii. p. 398) : — " In the adult 

 bird in summer the bill is coral-red, the point black, irides dark brown ; back and wings 

 ash-grey; outside web of the first primary slate-grey, the shaft white, inner web light 



* The old rule was that the hybrid offspring of species are sterile, aud the mongrel offspring of varieties are fertile. 



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