40 GENESIS OF THE ARIETID^E. 



form, with keels, channels, etc. 1 The metaplastic mode of development of the 

 progressive forms was again exchanged in the still higher and more specialized 

 but degenerate and geratologous forms, like Ad. Collenoti and the Oxynoticeran 

 series, for the cataplastic mode. The young more closely resembled the later 

 stages, and passed into them with less abrupt transitions, because of the action 

 of the law of acceleration, and the consequent omission of ephebolic character- 

 istics. The same process took place in each genus of the Arietidae, more or less 

 according to its place in the cycle, and in our descriptions of the species and 

 series the evidence will be given. 



Law of Acceleration. 2 



Whenever the character or form, whether healthy or pathological, became 

 fixed in the organism, it became at the same time subject to the law of ac- 

 celeration. Its previously transient and sporadic appearance ceased, and a 

 series came into being having this character, whatever it might be, constantly 

 reproduced in earlier stages of the species. 



The fixity of many characters on first appearance may be reasonably doubted, 

 but not their subsequent tendency to acceleration. The hollow keel of Oxyn. 

 oxynotum is a novel character, and it seems to be present in every mature speci- 

 men of the species. Nevertheless, in this and in all cases like this, there is a 

 dearth of evidence of a positive nature, and some specimens may yet be found 

 which did not possess it. There are however, examples without number like 

 the variable first appearance of the abdominal channel in Schlotheimia, of the 

 keel in Caloceras, and of the pilse in Arnioceras and Agassiceras. The young had 

 fold-like pilas in Schlotheimia, which crossed the abdomen. The single channel 

 which was subsequently produced is a progressive stage, arising partly from the 

 suppression of the folds along the median line, and partly from extra growth of 

 the pilse on either side of the abdomen. Sometimes the suppression of the pilse 

 along the median line of the abdomen never took place, the abdomen remaining 

 completely pilated throughout life. This occurred as a sporadic or varietal ephe- 

 bolic character in Schlot. catenata, but in the higher, later occurring, and more 



1 See Chapter III. of this memoir. 



2 I have used the term Law of Concentration in several recent essays, instead of, as formerly, "Law of 

 Acceleration," because my attention had been attracted to phenomena which showed that an animal having 

 accelerated development of characteristics did not necessarily have a quick development, but on the con- 

 trary might grow, so far as time is concerned, even more slowly than others of its own group. It became 

 essential, then, to get rid of the impression, which I had held in common with some other embryologists, 

 that an animal which skipped many characteristics of its ancestors, or of its own type, had necessarily a 

 quicker growth and development. Undoubtedly, in many instances, especially where acceleration is due to 

 pathological causes, such is the result; but this does not occur in all cases, or perhaps necessarily in any 

 case. In trying to introduce this idea I went too far, and in substituting the term "Concentration" for 

 " Acceleration " made a change which was not an improvement. In this memoir, therefore, I have returned 

 to the older and more appropriate term which stands at the head of this section. The term " abbreviated 

 development" is often used, as it has been by Balfour, for extreme examples of acceleration, and it also im- 

 plies no necessary relation of time. This term, however, was not invented to express a law of develop- 

 ment, and its author did not take into consideration the fact, that such cases are only the extreme expression 

 and the necessary result of a universal law of organic evolution. 



