454 THE POPULAR SCIENCE MONTHLY 



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 im- 

 portant factor in the production of new forms which have successively 

 occupied every vacant station with specially adapted species. 



Inheritance and Variation 



The vitally important facts of inheritance with variation was next 

 discussed, and their exact nature and universal application pointed out. 

 The laws of the frequency and the amount of variations, and 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 obtained 

 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 

 and comparisons, was, that the individuals of all species varied around 

 a 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 2,600 men, taken at 

 random, were measured, those about 5 feet 8 inches in height were 

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

 heights between 5 feet 6 inches and 5 feet 10 inches, while only ten 

 reached 6 feet 6 inches, or were so little as 4 feet 10 inches, and at 6 

 feet 8 inches and 4 feet 8 inches 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 through 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 that since the glacial epoch 

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

 the adaptation of every existing species to its environment was not 

 only special but general. The seasons changed from year to year, 

 but the extremes of change only occurred at long intervals, perhaps 

 of many centuries, with lesser, but still very considerable variations 

 twice or thrice in a century. It was by the action of these seasons 

 of extreme severity at long intervals, whether of arctic winters, or 

 summer droughts, that the very existence of species was endangered; 



