C. — aEOLOQY* 67 



tionary biologist, a species contains in itself and its environment the 

 possibility of producing its successor. The words ' its environment ' 

 are necessary, because a living organism cannot be conceived apart 

 from its environment. They are important, because they exclude from 

 the idea of organic evolution the hypothesis that all subsequent forms 

 were implicit in the primordial protoplast alone, and were manifested 

 either through a series of degradatio'ns, as when Thorium by successive 

 disintegrations transmutes itself to Lead, or through fresh develop- 

 ments due to the successive loss of inhibiting factors. I say ' a species 

 contains the possibility ' rather than ' the potentiality,' because we 

 cannot start by assuming any kind of innate power. 



Huxley, then, forty years ago, claimed that palaeontologists had 

 proved an orderly succession. To-day we claim to have proved evolution 

 by descent. But how do we prove it? The neontologist, for all his 

 experimental breeding, has scarcely demonstrated the transmutation 

 of a species. The palaeontologist cannot assist at even a single birth. 

 The evidence remains circumstantial. 



Recapitulation as^ Proof of Descent. 



Circumstantial evidence is convincing only if inexplicable on any 

 other admissible theory. Such evidence is, I believe, afforded by 

 palaeontological instances of Haeckel's law — i.e., the recapitulation by 

 an individual during its growth of stages attained by adults in the 

 previous history of the race. You 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 adulii resembles the adult 

 a^, and that the next preceding stage resembles a^, and so on; if this 

 applies viutatis 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 various 

 reasons, which we are learning to guard against. For example, 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 eccentricities 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 

 fived 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 R. G. Carruthers 

 in his paper on the evolution of Zaphrentis delanouei (1910, Quart. 

 Joum. Geol. Soc, Ixvi., S^S). 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 



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