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60 F W. HEADLEY [may 



a higher stage once reached can afford a foundation for one still higher} 

 since the former, the existing stage, will necessarily be the starting- 

 point for further modification." When reading his book some few years 

 ago, I passed over this passage, failing to realise its importance. Now, 

 having arrived at the same position independently, or, it may be, 

 having recurred unconsciously to what I read there, I look upon 

 the principle which Eimer enunciated as the clue to much that is 

 otherwise inexplicable in evolution. We see everywhere in the 

 organic world order and symmetry : hence it is difficult not to infer 

 that the course of evolution has some better guide than mere chance. 

 Some biologists find in external conditions, and in the creature's own 

 activities, guidance sufficient. But the influence of the environment 

 cannot shape a flower or thistle-down, or the samara of the maple. 

 Nor in these cases can exercise have anything to do with the matter. 

 We must find the guiding principle, then, in the organism itself, for 

 there is no third alternative. Are we to say, then, that the first 

 unicellular creatures contained in them in parvo all the complex 

 animals and plants that have since arisen ? That is unbelievable. 

 But there remains Eimer's principle. As I understand it, it is this. 

 Each step in advance decides within certain limits the next step. 

 A few examples will make this plain. I take any genealogical tree, 

 Haeckel's for instance ("Evolution of Man," vol. ii. p. 188), which is 

 generally acceptable enough for our present purpose, though, no doubt, 

 every biologist would prefer to draw out his own tree. Nearly at the 

 base we find amoebae, one-celled organisms with a nucleus. Even 

 at this very early stage there are very marked characteristics, notably 

 cell-structure and multiplication by fission. If evolution is to pro- 

 ceed much further, there must be increase in size. Now the 

 character which evolution has already impressed upon the amoeba 

 allows of this only by one method. When fission takes place, the 

 resultant cells may cling together, and then we have one of the 

 Metazoa or multicellular organisms. Take now a point more than 

 half way up the tree, where the Selachii, primitive fishes, are found. 

 From these we find branching off the various kinds of fishes and the 

 amphibians. Should the former course be followed there are paths 

 leading towards the ganoids, the osseous fishes, and the mud-fishes. 

 But development on amphibian lines, when once another alternative 

 has been chosen, has become an impossibility. It is true there may 

 be retrogression, but nature deprecates this, and even if a species 

 retraces its steps it does not retrace them very far. This being so, 

 every advance may be looked upon as a point gained. Every advance 

 cuts off certain possibilities, and opens up certain others. The process 

 may be compared to the system of case law, in which the interpre- 

 tation of the law by a judge, at any rate if upheld on appeal, becomes 

 binding on all judges who have to try similar cases. As we advance 



1 The italics are mine. 



