218 PROCEEDINGS OF THE AMERICAN ACADEMY. 



approached, certain losses of the characters of the adult took place, 

 or, if additional growths were acquired, these were of a peculiar kind. 

 These senile stages had been noticed by D"Orbigny and Quenstedt, 

 but these authors did not attempt to show that any correlations existed 

 between any stages of the ontogeny and the gradations occurring in the 

 full-grown forms during their evolution iu time, or what is called phylo- 

 geny. The oldest stage of the shell in Cephalopoda, Brachiopoda, and 

 Pelecypoda is commonly marked by a series of retrogressive changes, 

 which have been fully described elsewhere. These changes have a 

 similar nature to those found in the old age of man, but they are more 

 noticeable because they are recorded in the permanent characters of 

 the hardened shell. The old man returns to second childhood in mind 

 and body, and the shell of the cepbalopod has in old age, however dis- 

 tinct and hin'hly ornamented the adult, very close resemblance to its own 

 young. This resemblance is a matter of form and aspect only, since 

 there can be no close comparison in minute structure, nor functions 

 between organs and parts, at these two different ends of life. Such 

 analogies, however, have their own meaning, and are o-f great importance 

 when properly translated. 



In the first place they show that the cycle of life as manifested in man 

 is found also in the ontogeny of other animals and more perfectly in pro- 

 portion to the perfection of the record. They are consequently among 

 shell-bearing animals, especially those that carry their embryonic shells 

 and all their subsequent stages of development throughout their lives, 

 more perfect, more decisive, as well as more obvious, than in animals, like 

 the Vertebrata, which carry no such burden of hard dead parts, upon and 

 in which their stages of development are recorded. The cycle of the 

 ontogeny is, therefore, not only physiological, but it is also a definite 

 series of structural changes and is often accompanied by transformations 

 of remarkable and sometimes startling character. 



These retrogressive transformations in old age of the shells of Cepha- 

 lopoda, Brachiopoda, and Pelecypoda have been found to have decided 

 correlations with the adult characters of species that appear simultane- 

 ously or later in time. If one traces any group through its evolution 

 in time, it has, as stated by many authors, a period of rise called the 

 epacme, a second period of greatest expansion in numbers of forms and 

 species called the acme, and then usually a movement towards contraction 

 called the paracme. All three of these terms were first proposed by 

 riaeckel, who used them largely in a physiological or dynamical sense. 

 The epacme of any group, large or small, is usually a process of evolu- 



