126 MARINE BIOLOGICAL LABORATORY. 



years, and each year lays about twenty eggs. Hence 

 a pair of partridges may produce about four hundred 

 eoro:s in their Hfetime. This is at the rate of two thousand 

 in a hundred years. Yet, since the number of partridges 

 in the forest does not increase, three hundred and ninety- 

 eight of these eggs, or young, must be destroyed in 

 twenty years, while but two survive to take the place 

 of their parents. The eggs and young are destroyed 

 by beasts and birds of prey. If these enemies increased 

 very much in number, the partridge would become 

 extinct unless it laid more eggs. 



It would appear, then, that the partridge lays just 

 eggs enough to ensure the continuance of its race, 

 and this being accomplished, death removes it. Many 

 species have doubtless become extinct through the in- 

 sufficiency of their reproductive powers. The number 

 of offspring which, under ordinary conditions, would 

 have ensured perpetuation, has proved insufficient when 

 their enemies increased or the environment became 

 unfavorable. The supply must be equal to the de- 

 mand. 



Now for the eagle. The eagle is one of the most 

 powerful of birds, and builds its nest on such inac- 

 cessible cliffs that eggs and young are comparatively 

 safe from marauding animals. Many, however, are de- 

 stroyed by late frosts and snows. To be on the safe 

 side, let us fix the duration of life of the eagle as sixty 

 years, and of this ten years is spent in immaturity. 

 Hence there are fifty years of its life during which it 

 reproduces. If the eagle lays but two eggs a year, a pair 

 of eagles would produce one hundred during their life- 

 time. In a hundred years two hundred eggs against 



