262 



SCIENCE 



[N. S. Vol. LII. No. 1342 



previous history of the race. Tou all know 

 how this has been applied to the ammonites; 

 but any creatures with a shell or skeleton that 

 grows by successive additions and retains the 

 earlier stages unaltered can be studied by this 

 method. If we take a chronological series of 

 apparently related species or mutations, a}, a^, 

 a?, a*, and if in a* we find that the growth 

 stage immediately preceding the adult re- 

 sembles the adult a^, and that the next preced- 

 ing stage resembles a^, and so on; if this ap- 

 plies mutatis mutandis to the other species of 

 the series; and if, further, the old age of each 

 species foreshadows the adult character of its 

 successor; then we are entitled to infer that 

 the relation between the species is one of 

 descent. Mistakes are liable to occur for vari- 

 ous reasons, which we are learning to guard 

 against. For examiDle, the perennial desire of 

 youth to attain a semblance of maturity leads 

 often to the omission of some steps in the 

 orderly process. But this and other eccentrici- 

 ties affect the earlier rather than the later 

 stages, so that it is always possible to identify 

 the immediate ancestor, if it can be found. 

 Here we have to remember that the ancestor 

 may not have lived in the same locality, and 

 that therefore a single cliff-section does not 

 always provide a complete or simple series. 

 An admirable example of the successful search 

 for a father is provided by E. G. Carruthers 

 in his paper on the evolution of Zaphrentis 

 delanouei^ Surely when we get a clear case 

 of this kind we are entitled to use the word 

 " proof," and to say that we have not merely 

 observed the succession, but have proved the 

 filiation. 



It has, indeed, been objected to the theory 

 of recapitulation that the stages of individual 

 growth are an inevitable consequence of an 

 animal's gradual development from the em- 

 bryo to the adult, and therefore prove nothing. 

 Even now there are those who maintain that 

 the continuity of the germ-plasm is' inconsist- 

 ent with any true recapitulation. Let us try 

 to see what this means. Take any evolution- 

 ary series, and consider the germ-plasm at any 

 early stage in it. The germ, it is claimed, con- 



4 1910, Quart. Jour. Geol. Soc, LXVI., 523. 



tains the factors which produce the adult 

 characters of that stage. ISTow proceed to the 

 next stage of evolution. The germ has either 

 altered or it has not. If it has not altered, the 

 new adult characters are due to something 

 outside the germ, to factors which may be in 

 the environment but are not in the germ. In 

 this case the animal must be driven by the 

 inherited factors to reproduce the ancestral 

 form; the modifications due to other factors 

 will come in on the top of this, and if they 

 come in gradually and in the later stages of 

 grovrth, then there will be recapitulation. 

 There does not seem to be any difficulty here. 

 Tou may deny the term " character " to these 

 modifications, and you may say that they are 

 not really inherited, that they will disappear 

 entirely if the environment reverts to its orig- 

 inal condition. Such language, however, does 

 not alter the fact, and when we pass to subse- 

 quent stages of evolution and find the process 

 repeated, and the recapitulation becoming 

 longer, then you will be hard put to it to imag- 

 ine that the new environment produces first 

 the effects of the old and then its own partic- 

 ular effect. 



Even if we do suppose that the successive 

 changes in, say, an ammonite as it passes from 

 youth to age are adaptations to successive en- 

 vironments, this must mean that there is a 

 recapitulation of environment. It is an ex- 

 planation of structural recapitulation, but the 

 fact remains. There is no difficulty in sup- 

 posing an individual to pass through the same 

 succession of environments as were encount- 

 ered in the past history of its race. Every 

 common frog is an instance. The phenomenon 

 is of the same nature as the devious route 

 followed in their migrations by certain birds, 

 a route only to be explained as the repetition 

 of past history. There are, however, many 

 cases, especially among sedentary organisms, 

 which can not readily be explained in this way. 



Let us then examine the other alternative 

 and suppose that every evolutionary change is 

 due to a change in the germ — how produced we 

 need not now inquire. Then, presumably, it 

 is claimed that at each stage of evolution the 

 animal will grow from the egg to the adult 



