170 



SCIENCE. 



[N. S. Vol. V. No. 109. 



Lastly, it lias been found that at the be- 

 ginning of the evolution of any stock the 

 progress was not only very rapid, but the 

 departures in structures much more marked 

 between the diverging lines of different 

 species, genera or families, and so on, than 

 those that subsequently occurred in any 

 one of these. This rapidity of expansion 

 is also marvellously sudden in every series 

 near its point of origin, and it is equally so 

 in the whole animal kingdom, which ap- 

 pears with the larger proportion of all its 

 principal divisions in the eai'liest known 

 fossil-bearing rocks. Each series or type 

 appears to have had a more or less free iSeld, 

 and its first steps in evolution were obviously 

 not affected by natural selection. Subse- 

 quently, in the acme of the same series or 

 type, the departures became less marked, 

 and the divergences took place in less im- 

 portant structures ; in other words, as stated 

 above, the evolution is slower. 



On the other hand, after the acme is 

 passed and the paracme sets in, there is a 

 sensible quickening of evolution during de- 

 cline. 



Phylogerontic forms become more and 

 more numerous, and there are wider de- 

 partures in the structures from the acmatic 

 forms than any of the divergences that oc- 

 cur within the acmatic forms themselves. 



The hopping, skipping and, at last, the 

 jumping begins in the extremes of the series, 

 so that it becomes difficult, as has been shown 

 by the author in a number of series and by 

 Cope when giving illustrations of the action 

 of the law of tachygenesis, to connect one of 

 these extreme forms with its nearest con- 

 geners. 



The characters of the cycle in the ontog- 

 eny are here again similar to those of the 

 phylogeny ; thus the final substages of the 

 gerontic stage are wider departures from the 

 ephebic substages than these are among 

 themselves and when compared with each 

 other. The analogy of the old with the 



young shows this most conclusively and 

 with the similarity of phylogerontic forms 

 in the same stock occurs a parallelism in 

 the phylogeny. 



In fact, there is no end to the homological 

 and analogical similarities and parallelisms 

 of ontogeny and phylogeny wherever both 

 are found complete. 



There are types in which the ontogeny is 

 incomplete, as among insects and other 

 purely seasonal animals, and in these it 

 becomes difEicult, if not impracticable, to 

 study the gerontic stages, and thus translate 

 the phylogerontic types if they occur. 

 These same types, and others also, present 

 difficulties in their larval stages, owing to 

 their indirect modes of development, which 

 have been discussed by the author in In- 

 secta and other publications, and need only 

 be referred to here. 



One of the bearings of these researches 

 is of interest on account of the discussions 

 between biologists, geologists and mathe- 

 maticians with regard to the length of time 

 that life has existed on this planet and the 

 bearing of this upon calculations with re- 

 gard to the age of the earth. It cannot be 

 assumed that the time ratio was the same 

 during the eozoic or pre-Paleozoic as during 

 the Paleozoic or the Mesozoic, so far as the 

 evolution of forms is concerned. The evi- 

 dence is very strong that great structural 

 difi'erences were evolved much more quickly 

 in these early times, and the probabilities 

 are that the progressive steps of the evolu- 

 tion of the primitive types of organisms 

 took place with a rapidity unexampled in 

 later ages. If the laws of bioplastology 

 are true the evolution of these forms must 

 have occurred more quickly than those of 

 their descendants, except perhaps some iso- 

 lated phylogerontic types and phylopathic 

 forms.* 



* The plirase 'evolution by saltation' has been used 

 for the sudden appearance of divergent types by sev- 

 eral authors, first by Dr. W. H. Dall; but this seems 



