158 DARWINISM TO-DAY. 



"During the last six years my friend, Mr. Herbert Thompson, and 



I have studied in some detail the state of this character in the 



Weldon's ex- small shore-crabs which swarm on the beach below 



periments on the laboratory of the Marine Biological Association, 



Carcinus. at Plymouth. 



"I will show you that in those crabs small changes in the size of 

 the frontal breadth do, under certain circumstances, affect the death- 

 rate, and that the mean frontal breadth among this race of crabs is, 

 in fact, changing at a rate sufficiently rapid for all the require- 

 ments of a theory of evolution. 



"In Table IV [omitted], you see three determinations of the 

 mean frontal breadth of these crabs, expressed in terms of the 

 carapace-length, taken as 1,000. You see that the mean breadth 

 varies very rapidly with the length of the crab, so that it was neces- 

 sary to determine it separately in small groups of crabs, such that 

 the length of no two crabs in a group differed by more than a 

 fifth of a millimetre. The first column of the table shows you the 

 mean frontal breadth of twenty-five such groups, between 10 and 15 

 millimetres long, collected in 1893. These crabs were measured by 

 Mr. Thompson. The second column shows you the mean frontal 

 breadth in twenty-five similar groups of crabs, collected in 1895, and 

 also measured by Mr. Thompson. You see that in every case the 

 mean breadth in a group of crabs collected in 1895 is less than it 

 was in crabs of the same size collected in 1893. The third column 

 contains the result, so far as it is yet obtained, of my own measure- 

 ment of crabs collected this year. It is very incomplete, because 

 the 1895 crabs were collected in August and September, and I was 

 anxious to compare them with crabs collected this year at the same 

 season, so that there has not yet been time to measure the whole 

 series. The measurements are sufficient, however, to show that 

 the same kind of change has taken place during the last three 

 years as that observed by Mr. Thompson in the interval between 

 1893 an d 1895. Making every allowance for the smallness of the 

 numbers so far measured this year, there is no doubt whatever that 

 the mean frontal breadth of crabs from this piece of shore is 

 considerably less now then it was in 1895 among crabs of the 

 same size.* 



"These results all relate to male crabs. The change in female 



* I shall, of course, consider it my duty to justify this statement 

 by more extensive measurement as soon as possible. In the mean- 

 time I may say that I have measured other small groups of crabs, 

 male and female, from the same place, at different seasons of the 

 years 1896-98, and the results agree with those recorded in the 

 table. 



