OO Elliot, Inhen'fance of Acquired Chatacfers. [January 



slow in arriving. It is difficult to ascertain the age to which 

 wild creatures are capable of attaining, but as the I'esults of our 

 only means of observation, viz., animals in captivity, it is found 

 that Eagles live nearly one hundred years.* Ravens live nearly 

 as long. Parrots have reached nearly one hundred years. Mag- 

 pies live twenty years, and small birds, as the Nightingale, Black- 

 bird, etc., from ten to twenty years. As a rule birds of long 

 lives lay but few eggs at a time, and allowing for the destruction 

 of the eggs from numerous causes, and the mortality among the 

 young, it is estimated by Weismann,f arguing on the lines of 

 Darwin and Wallace, that given the fertility and average of life 

 of a species, a calculation can be made of the number of those 

 reaching maturity. A species living ten years and laying twenty 

 eggs each year, only two of its young in that time will reach matur- 

 ity, if the number of individuals in the species is to remain con- 

 stant. Or if the duration of life of an eagle is sixty years, and it 

 reaches maturity at ten years, and then lays two eggs a year, 

 out of the one hundred eggs laid only two will develop into adult 

 birds. Weismann considers that this calculation rather under- 

 than over-estimates the proportion of mortality among the young. 

 However this may be, or whether we may altogether accept his 

 calculations as correct, or warranted by recorded data, it appears 

 to be a recognizable fact that the greater the age of an animal tlie 

 slower it reproduces, and is the less able to originate divergences 

 from those characters it received at its birth. The influences of 

 the environment would be slower in their effects on a long-lived 

 individual, and changes from its type, or starting point, corres- 

 pondingly delayed. We must therefore look for variations and 

 departures from the types more among those species with shorter 

 lives, and greater number of offspring, where the influences of 

 environment and natural selection are able to produce their legit- 

 imate eflects in the briefest periods of time. 



What is the result of the effect of environment on individuals.? 

 It provides them with the means of succeeding in the strug- 

 gle for life. Dwellers of deserts, or desert-like districts, are pale 

 in coloration and assimilate the ground, and so escape detec- 

 tion. Humidity and rainfall cause a melanistic phase of plumage. 



* Brehm, Leben der Vogel, p. 72. Weism. Essays, p. 37. 

 t Duration of Life, Essays, 2d ed. p. 13. 



