August 30, 1901.] 



SCIENCE. 



319 



lution are inheritance and variation. In 

 the new century careful and quantitative 

 studies will be made on these factors, "We 

 shall get at quantitative expressions of 

 the more complicated forms of heritage in 

 tlie same way as Galton has given us an 

 expression of a simple form of inheritance. 

 We shall hope to understand why some 

 qualities blend and others refuse to do so. 

 We shall learn the laws of mingling of 

 qualities in hybrids and get an explanation 

 of the monstrosities and the sterility which 

 accompany hybridization. What we call 

 reversion and prepotency will acquire a 

 cj'tological explanation, and it may be that 

 the theory of fertilization will be seriously 

 modified thereby. When we can predict 

 the outcome of any new combination of 

 germ plasms then, indeed, we shall have got 

 at the laws of inheritance. 



As for the other factor, that of variation, 

 I anticipate interesting developments in our 

 knowledge of its laws and of its causes. 

 The methods by which this knowledge is 

 to be acquired are doubtless comparative 

 observation, experimentation and a quan- 

 titative study of results. Within tlie last 

 decade a profound student of variation 

 (Bateson) has declined to discuss its 

 causes, holding that we had no certain 

 knowledge of them. Even the categories 

 of variation are still unenumerated. The 

 science of variation is therefore one of those 

 that we may hope to see established in this 

 century. I feel convinced that statistical 

 studies are first of all necessary to lay the 

 foundations of the science. 



As an illustration of an application of 

 statistics to evolution studies I will give 

 some account of my work during the past 

 two years on the scallop of our east coast, 

 Pecten irradians. 



Peden irradians is a bivalve mollusc of 

 flattened, lenticular form, that inhabits our 

 coast from Cape Cod southward. The 

 Cape Cod limit is a rather sharp one, bub 



southward our scallop passes gradually 

 into the closely related forms of the South 

 American coast. This fact would seem to 

 indicate its southerly origin. To get light 

 on the evolution of the group, I have 

 studied and measured over 3,000 shells, 

 chiefly from four localities: (1) Cold 

 Spring Harbor, Long Island; (2) More- 

 head, North Carolina ; (3) Tampa, Florida, 

 and (4) the late Miocene or early Pliocene 

 fossils of the ISTansemond River. The fos- 

 sil shells, to which I shall frequently refer, 

 were found imbedded in the sand at Jack's 

 Bank, one mile below Suffolk; Virginia. 

 The bank rises to a height of 25 to 30 feet. 

 Shells were obtained from three layers, 

 respectively, one foot, six feet and 15 feet 

 above the base of the bluff. Of course, the 

 upper shells lived later than the lower 

 ones and may fairly enough be assumed to 

 be their direct descendants. The time in- 

 terval between the upper and lower levels 

 cannot be stated. As I have measured 

 sufficient shells from the bottom and top 

 layers only I shall consider them chiefly. 

 I wished to get recent Pedens from this 

 locality, but the nearest place where they 

 occur in quantity is Morehead, l^orth Caro- 

 lina. These Pedens may therefore stand as 

 the nearest recent descendants of the Pec- 

 tens of the ISTansemond River. 



The Peden shells have a characteristic 

 appearance in each of the localities studied. 

 After you have handled them for some time 

 you can state in 95 per cent, of the cases 

 the -locality from which any random shell 

 has come. First of all, the shells differ in 

 color, especially of the lower valve. In the 

 specimens from Cold Spring Harbor this is 

 a dirty yellow ; from Morehead, yellow to 

 salmon ; from Tampa white through clear 

 yellow to bright salmon. Second, the an- 

 tero-posterior diameter of the shell becomes 

 relatively greater than the vertical diameter 

 as you go north". Thus, the antero-posterior 

 diameter exceeds, on the average, the dorso- 



