322 



SCIENCE. 



[N. S. Vol. XIV. No. 348. 



work I have been engaged on during the 

 past year. I have been counting the num- 

 ber of rays in recent Peeten irradians from 

 various localities and have obtained in some 

 cases evident skewness in the frequency 

 polygons. To see what phylogenetic mean- 

 ing, if any, this skewness has I sought to 

 get a series of late fossils. After careful 

 consideration I was led to go to the Nanse- 

 mond Eiver for the late Tertiary fossils 

 found there and already referred to. These 

 served my purpose admirably. We may 

 now compare the average number of rays 

 from the two extreme layers at Jack's 

 Bank and at Morehead with the indices of 

 skewness of the frequency polygons from 

 the same localities. 



This series is instructive in that it tells 

 us that the gradual reduction in the num- 

 ber of rays has been accompanied at each 

 preceding stage by a negative skewness. 

 This skewness was thus ^prophetic of what 

 was to be. The skew condition of the 

 frequency polygon we may attribute to a 

 selection taking place at every stage, and 

 the interesting result appears that the 

 selection diminishes in intensity from the 

 earliest stage onward. It is as though 

 perfect adjustment were being acquired. 

 If adjustment were being perfected we 

 might expect a decrease in the variability in 

 the rays at successive periods. And we do 

 find such a decrease. This is indicated in 

 the last column where <t stands for the in- 

 dex of variability. From this column it ap- 

 pears that the variation in the number of 

 rays has diminished from 1.24 rays in the 

 Miocene to 0.81 rays in recent times. This 

 fact again points to an approach to perfec- 

 tion and stability on the part of the rays. 



Just why or wherein the reduced number 

 of rays is advantageous I shall not pretend 

 to say. It is quite possible that it is not 

 more advantageous, but that there is in the 

 phylogeny of Peeten irradians an inherent 

 tendency towards a reduction in the num- 

 ber of multiple parts. As a matter of fact 

 there are other Pectens in which the num- 

 ber of rays is less even than in irra- 

 dians. 



The reduction in the variability of the 

 rays with successive geological periods has 

 another interest in view of the theory of 

 Williams and of Eosa, according to which 

 evolution and differentiation have of ne- 

 cessity been accompanied by a reduction in 

 variability. Evolution consists, indeed, of 

 a splitting off of the extremes of the range 

 of variation, so that in place of species with 

 a wide range of variability we have two or 

 three species each with a slight range of 

 variability. In the particular case in hand, 

 however, it is not certain that the lower 

 Jack's Bank form-unit (named Peeten ebor- 

 eus by some one) has given rise to any other 

 form than something of which Peeten ' ir- 

 radians ' of Morehead is a near representa- 

 tive. The evidence indicates that the re- 

 duced variability is solely the effect of the 

 skewing factors. 



The upshot of this whole investigation into 

 the biological significance of skew variation 

 is then this : Skewness is sometimes remin- 

 iscent and sometimes prophetic. In our 

 present state of knowledge it is not possible 

 by inspecting a single skew curve to say 

 which of the two interpretations is correct 

 in the given case. But by a comparison of 

 the frequency curves of allied form-units 

 the state of affairs can usually, as in the ex- 

 amples given, be inferred. A method of 

 interpreting the single skew curve is a dis- 

 covery for the future. 



I realize that I have been bold, not to say 

 rash, in this attempt to forecast the zo- 

 ology of the twentieth century. I suppose, 



