1918] Barrows: Skeletal Variations in the Genus Peridinium 463. 



20. As might be expected in conformity with Child's law of meta- 

 bolic gradients, the greatest morphologic activity and tendency for 

 fundamental change is located in the anterior portion of the shell, 

 while the posterior portion is conservative and carries modifications 

 which may be due not so much to an internal stimulus as to more 

 direct but minor responses to an outside stimulus. It seems probable 

 that in these organisms, as well as those upon which Child worked, 

 it is the strong unceasing impact of the environment upon the anterior 

 end of the organism which has made it the more active and the more 

 likely to respond with profound morphological modifications. 



21. In each varying pattern the suture which marks the juxtaposi- 

 tion of two of the quartette of plates involved in the given pattern and 

 which therefore becomes characteristic for that pattern varies about 

 a mode of its own. Intervening stages of the gradual shortening of 

 such a suture in a supposed transition to the alternate pattern beyond 

 the established limits of variation for the suture are unknown. Each 

 pattern is separate and distinct from its alternate and one pattern 

 by no means grades into the other. There is, thus, a definite gap 

 between variations of the alternate members of a pair of plate patterns 

 at each of the four variable regions, as examined intensively for the 

 species, P. divergens Ehrbg., and a similar gap apparent between cer- 

 tain rare changes in plate pattern observed in four other species. 



In the experience of the writer no case has been found in which 

 four sutures actually meet at one point. Cases which at first sight 

 appeared as such, upon closer analysis in a favorable position have 

 been resolved into two junction points of three sutures each separated 

 by a short suture which is one of the two critical sutures for the pair 

 of alternate plate patterns possible by the rearrangement of plates in 

 the given region. 



22. Mutations appear in two ways in this genus: (1) by the svid- 

 den change of plate pattern within a species as found occasionally in 

 the species just referred to above ; and ( 2 ) by the introduction of new 

 plates from time to time in the history of the family to which the 

 genus, Peridinium, belongs. These new plates become new characters 

 abruptly introduced, and each passes through a process of develop- 

 ment, involving enlargement and various adjustments with the ad- 

 jacent plates of the shell. 



23. It seems probable then if the species of this genus are in verity 

 phylogentically related : that one species arose from a previously 

 existing species by a mutation, involving an abrupt addition of a new 



