HYATT. — ORGANIC CYCLES. 221 



Store of vitality, and both must progress and retrogress, complete a cycle 

 and finally die out, in obedience to the same law. 



All of these views can be well supported, but, whatever may be the 

 true explanation, it is obvious that there are plenty of paracmatic types, 

 which, in their full-grown and even in their neanic stages, correlate in 

 characters and structures with the characters and structures that one 

 first finds in the transient gerontic stages of acmatic forms of the same 

 type. These can, therefore, be truthfully and accurately described as 

 phylogerontic in the phylum. 



In other words, one is able to apply gerontic changes in the ontogeny 

 to the deciphering of the true relations, the arrangement and classification 

 of forms occurring in the paracme, just as Agassiz's law of palingenesis 

 can be used to explain the relations of the links in the chain of being 

 forming the epacme of groups. 



The cycle of the ontogeny is, therefore, the individual expression and 

 abbreviated recapitulation of the cycle that occurs in the phylogeny of 

 the same stock; and, while the embryonic, nepionic, and neanic stages- 

 give us, in abbreviated shape, the record of the epacme, the gerontic 

 stages give, in a similar manner, the history of the paracme. 



The difference between the nature of the two records is, however, neces- 

 sarily as great as between the beginnings and the endings of existence. 

 The successive stages of the individual are derived from the past, and 

 simply point backwards along the track traversed by the phylum ; the 

 changes of the gerontic stage, on the other hand, point to the future, 

 and are prophetic of what is to come in the decline of the type. The 

 retrogressive decline of the individual and that of its type are along 

 parallel lines, and the two are in direct correlation, so that the former 

 becomes an abbreviated index of the latter. 



One of the most useful results of these studies has been the method 

 of work developed, the mode of study by series. To follow it out suc- 

 cessfully, one must trace the terms of series from the first or most 

 primitive grade to the last, through perhaps long periods of time, and, if 

 upon the same level, through many gradations of structure. 



The histologist or embryologist picks out a convenient form here and 

 there for thoi'ough investigation, but does not seem as yet to see the 

 importance of the point of view here insisted upon, viz. that the only 

 method of getting at the correlations of ontogeny and phylogeny is by 

 following out the history of representative series of genetically connected 

 embryos, and the same is true of the experimentalist. "While, conse- 

 quently, their results have been in the highest degree instructive and 



