1909] on the World of Life. 425 



length to the cases of the bison and the passenger pigeon in North 

 America, and the lemmings of Scandinavia. In the insect tribes still 

 more rapid powers of increase exist. The common flesli-fly goes 

 through its complete transformations from Qgg to perfect insect in 

 two weeks ; and Linnaeus estimated that three of these flies could 

 €at up a dead horse as quickly as a lion. 



It is these enormous powers of rapid increase that have ensured 

 the continuance of the various types of existing life from the earliest 

 geological ages in unbroken succession ; while it has also been an 

 important factor in the production of new forms which have suc- 

 oessively occupied every vacant station with specially adapted species. 



Inheritance and Variation. 



The vitally hnportant facts of inheritance with variation was 

 next discussed, and their exact nature and universal application 

 pointed out. The laws of t\iQ frequency midi the amount of variations, 

 iind their occurrence in all the various parts and external organs of 

 the higher animals, was illustrated by a series of diagrams. These 

 showed the actual facts of variation in adult animals of the same sex 

 o])tained at the same time and place, which had been carefully 

 measured in numbers varying from twenty to several thousand 

 individuals. 



The general result deduced from hundreds of such measurements 

 ^nd comparisons, was, that the individuals of all species varied around 

 ii mean value — that the numbers became less and less as we receded 

 from that mean, and that the limit of variation in each direction was 

 soon reached. Thus, when the heights of 2600 men, taken at 

 random, were measured, those about 5 ft. 8 in. in height were found 

 to be far the most numerous. About half the total number had 

 heights between 5 ft. 6 in. and 5 ft. 10 in., while only 10 reached 

 6 ft. 6 in., or were so little as 4 ft. 10 in., and at 6 ft. 8 in. and 4 ft. 

 8 in. there were only one of each. 



The diagrams from the measurements of various species of birds 

 and mammals were shown to agree exactly in generaL character ; and 

 the further fact was exhibited by all of them, that the parts and 

 organs varied more or less independently, so that the wings, tails, 

 toes, or bills of birds were often very long, while the body, or some 

 other part was very short, a point of extreme importance, as supplying 

 ample materials for adaptation througli natural selection. 



The Law of Natural Selection. 



The next subject discussed was the nature and mode of action 

 of natural selection. It was pointed out tliat since the glacial 

 epoch no decided change of species had occurred. This showed us 



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