HYATT. — ORGANIC CYCLES, 217 



life from and through living matter only. There are two different 

 manifestations of Awassiz's law, which Haeckel defined and named 

 'palingenesis' and ' oaMiogenesis,' the former referring to the ordinary 

 or regular mode in which the characteristics of ancestors are repeated 

 in the development of the individual, and the other to what is frequently 

 called the abbreviated mode, etc. 



These two modes are by no means all, but at present only the first 

 or simplest manifestations of the phenomena need be treated of. This, 

 or what Haeckel very appropriately calls ' palingenesis,' was what Louis 

 Agassiz had studied, and, so far as all the essential facts were concerned, 

 thoroughly understood, and it was this that he taught his students, so that 

 it became, at any rate in my own case, the foundation of all my subse- 

 quent work in determining the mutual relations of forms. If then, as I 

 have proposed in former publications, the term ' law of palingenesis ' be 

 adopted, this expressly states just what Louis Agassiz discovered. 



Observations upon this ground made especially upon Cephalopoda have 

 led to the discovery of correlations between the latter or epembryonic 

 stages and the adult stages of extinct ancestors, which have greatly en- 

 larged the field of application of Agassiz's law of palingenesis, and given 

 it an exactitude that has made it of surpassing importance in the study 

 of evolution. Beecher has been able to point out the single species of 

 Brachiopod from which the whole of the vast number of distinct forms 

 of this great group have originated. He has established this fact not 

 only by showing that the young of the existing and fossil forms all repeat 

 more or less at one stage the form of the adult of the initial species, but 

 has also found a very near affine of this single ancestral species as a 

 fossil appearing in one of the earliest of fossil-bearing formations. 



Dr. R. T. Jackson has done the same work for the Aviculoids among 

 the Pelecypoda, tracing all to one genus, JV^iicula, and has treated the 

 Echinoidea in the same way, tracing them by the use of Agassiz's law to 

 the senus JBothriocidaris. 



Although the evidence is perhaps less conclusive with reference to the 

 ancestor of Cephalopoda as a whole, this class has furnished the means 

 of showing the action of this law in smaller groups with great accuracy. 

 It has been possible to trace the origin of a number of smaller groups to 

 single ancestors within the class by carefully studying the correlations of ' 

 the epembryonic stages with the adults of the same group that have pre- 

 ceded them in time, and this study has also led to further discoveries. It 

 has been found that the new characters were first introduced in the later 

 stages of ontogeny, usually in the full-grown stage ; then, as old age 



