82 



SCIENCE 



[N. S. Vol. XL. No. 1020 



degrees of its development in a large num- 

 ber of individuals. He found by this means 

 that characters simply follow the laws of 

 probability. They vary around an average 

 condition in two directions, of increase 

 and decrease, but precisely thereby this 

 variability excludes the production of a 

 new character. Darwin tried to derive the 

 one from the other, whilst the theory of 

 mutations recognizes the almost diametri- 

 cally opposed nature of the two phenomena. 



A last objection has been brought for- 

 ward by the study of the age of the earth. 

 Physicists as well as astronomers have re- 

 fused to accept the theory of slow evolu- 

 tion as the time required by Darwin 

 in connection with his ideas, seemed by far 

 too long. A man's life would not suffice 

 to see the changes, which, after him, would 

 be necessary to produce a single step in 

 the line of evolution. The differentiation 

 of a flower or of a seed would require 

 millions of years if it went on so slowly, 

 and the development of the whole organism 

 of a plant, and still more so that of the 

 higher animals, would obviously require a 

 vastly larger amount of time. Darwin has 

 calculated the necessary time for the evolu- 

 tion of the whole animal and plant kingdom 

 on the assumption of slow and almost im- 

 perceptible changes, and estimated it to be 

 at least equal to some thousands of millions 

 of years. 



But our globe can not be as old as that. 

 There is quite a large number of arguments 

 which allow us to estimate the age of the 

 earth with a sufficient degree of accuracy, 

 and they all point, unanimously, to a period 

 of only some twenty or forty millions of 

 years. This number is evidently far too 

 small for the explanation given by Darwin 

 and in consequence thereof it has always 

 been considered as one of the most decisive 

 arguments against the theory of slow and 

 gradual evolution. 



In order to estimate the age of the earth 

 different phenomena may be used. First 

 the separation of the moon, secondly the 

 solidification of the earth's crust, then the 

 condensation of the aqueous vapor and the 

 formation of oceans. The quantity of salt 

 dissolved in these oceans and the thickness 

 of the geological layers, especially those of 

 a calcareous nature, afford further argu- 

 ments. 



According to George Darwin the moon 

 was separated from our globe about 56 

 millions of years ago. The age of the solid 

 crust has been calculated by Lord Kelvin 

 from the increase of the temperature in 

 deep mines. In some regions the tempera- 

 ture is seen to increase about one degree 

 for every fifty meters; in others, however, 

 one degree for a hundred meters. On the 

 average the considerations of Lord Kelvin 

 gave an age of twenty to forty millions of 

 years for the solid crust of the earth. 



The quantity of salt obviously increases 

 in the oceans on account of the salt added 

 by the rivers and of the evaporation of the 

 water. The total quantity of this salt has 

 been calculated and the quantities of the 

 yearly supply of water are known for all 

 the larger streams, as well as their percen- 

 tage of salt. From these data we may cal- 

 culate the annual increase of salt in the 

 oceans and find how many years would be 

 required for our present rivers to accumu- 

 late all the salt now found in the seas. 

 According to Joly, about ninety millions of 

 years would be necessary. But obviously 

 the rivers must exhaust the grounds which 

 they drain, and formerly these must there- 

 fore have been much richer in salts. This 

 consideration must lead us to diminish the 

 number of years required in a very sen- 

 sible manner. 



The age of the geological strata has been 

 deduced from their thickness and the 

 velocity of the process of sedimentation. 



