1 88 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



whose stages were once the exact parallel of a permanent 

 lower form, the condition of inexact parallelism. As all the 

 more comprehensive groups present this relation to each 

 other, we are compelled to believe that acceleration has been 

 the principle of their successive evolution during the long 

 ages of geologic time. Each type has, however, its day of 

 supremacy and perfection of organism, and a retrogression 

 in these respects has succeeded. This has, no doubt, 

 followed a law the reverse of acceleration, which has been 

 called 7'eta?'dation. ^y the increasing slowness of the 

 growth of the individuals of a genus, and later and later 

 assumption of the characters of the latter, they would be 

 successively lost." This statement of Cope might apply 

 equally well to unequal acceleration of characters, but in 

 another part of this same work he gives a clearer statement: 

 " Where characters which appear latest in embryonic his- 

 tory are lost, we have simple retardation, that is, the animal 

 in successive generations fails to grow up to the highest 

 point of completion, falling further and further back, thus 

 presenting an increasingly slower growth in the special 

 direction in question." (Cope, 1887, p. 13.) 



These remarks of Cope were based on abstract reasoning, 

 but it is possible to bring up some striking cases in support 

 of the theory, notably among the brachiopods. Fischer and 

 Oehlert (1892) have shown that while brachiopods go through 

 many metamorphoses in individual evolution, and while each 

 species is usually constant in the stages it goes through, it 

 often happens that the individual is arrested in development, 

 never reaching the full generic development of the mature 

 stage. The individual then begins to reproduce its kind 

 before maturity is reached, and tends to give rise to a stock 

 that never reaches the full generic evolution of its ancestors. 

 Dr. C. E. Beecher (1893^) has well described this: "In each 

 line of progression in the Terebratellid^ the acceleration of the 

 period of reproduction, by influence of environment, threw 

 oft genera which did not go through the complete series of 

 metamorphoses, but are otherwise fully adult, and even may 

 show reversional tendencies due to old age ; so that nearly 



