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SCIENCE. 



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



appropriately calls ' palingenesis,' was what 

 Louis Agassiz had studied and, so far as all 

 the essential facts were concerned, thor- 

 oughly 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 subsequent 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 enlarged the field of application of 

 Agassiz's law of palingenesis and given it 

 an exactitude that has made it of surpass- 

 ing 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 

 affinity of this single ancestral species as a 

 fossil appearing in one of the earliest of 

 the fossil-bearing formations. 



Dr. R. T. Jackson has done the same 

 work for the Pelecypoda, tracing all to one 

 genus, Nueula, and has treated the Echino- 

 dermata in the same way, tracing them by 

 the use of Agassiz's law to the genus Both- 

 riocidaris. 



Although the evidence is perhaps less 

 conclusive with reference to the ancestor of 

 Cephalopoda as a whole, this class has fur- 

 nished the means of showing the action of 

 this law in smaller groups with great accu- 

 racy. It has been possible to trace the origin 

 of a number of smaller groups to single an- 



cestors within the class by carefully studying 

 the correlations of the epembryonic stages 

 with the adults of the same group that have 

 preceded them in time, and this study has 

 also led to further discoveries. It has beeB 

 found that the new characters were first 

 introduced in the later stages of ontogeny, 

 usually in the full-grown stage; then, as old 

 age approached, certain losses of the charac- 

 ters of the adult took the place, or, if addi- 

 tional 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 

 occun-ing in the full-grown forms during 

 their evolution in time, or what is called 

 phylogeny . The oldest stage of the shell in 

 Cephalopoda, Brachiopoda and Pelecypoda 

 is commonly marked by a series of retro- 

 gressive 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 charac- 

 ters of the hardened shell. The old man 

 returns to second childhood in mind and 

 body, and the shell of the cephalopod has 

 in old age, however distinct and highly 

 ornamented the adult, very close resem- 

 blance to its own young. This resemblance 

 is a matter of form and aspect onlj% 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 of 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 pefectly in proportion to the perfec- 

 tion of the record. They are consequently 

 among shell-bearing animals, especially 

 those that carry their embryonic shells and 



