TRANSACTIONS OF SECTION D. 623 



application of this principle to the evolution of the teeth have been raised by Mr. 

 E. B. Poulton :— 



A. To the objection that the teeth are entirely formed before piercing the gum, 

 and that use produces an actual loss of tissue as contrasted with the growth of 

 bone, it may be said that by our theory, it is not the grovrth itself, but the reactions 

 which produce this growth in the living tissue, which we suppose to be trans- 

 mitted. 



B. To the objection that this proves too much, that the cusps thus formed 

 would keep on growing, it may be said (a) that in the organism itself these reac- 

 tions occur least in the best adapted structures. This proposition is difficult to 

 demonstrate in the case of the teeth, but may be readily demonstrated in what are 

 known as the phenomena of displacement in the carpals and tarsals where growth 

 has a direct ratio to impact and strain, (b) In the organism itself growth does not 

 take place beyond the limits of adaptation; there is, therefore, no ground for the 

 supposition that overgrowth will take place by transmission, (c) Either by the 

 selection or Lamarckian theory development, is held in check by competition between 

 the parts ; there is a limit to the nutritive supply ; in the teeth, as elsewhere, the 

 hypertrophy of one part necessitates atrophy of another. 



C. A general objection of considerable force is that we find other adaptations, 

 equally perfect, in which the Lamarckian principle does not apply ; why then 

 invoke it here ? To this it may be said that there is no theoretical difficultv in 

 supposing that while natural selection is operating directly upon variations of the 

 first class, the Lamarckian principle is producing variations of the second class, 

 and while selection does explain the former, it falls far short of explaining the 

 latter. 



Z). Finally, if Weismann succeeds in invalidating the supposed proofs of the 

 Lamarckian principle derived from pathology and mutilations, this will not afiect 

 the argument from palaeontology and comparative anatomy, for these proofs 

 involve two elements which are not in our theorem : (a) immediate transmission 

 of characters ; (b) transmission of characters impressed upon the organism and not 

 self-acquired. 



3. As regards the adequacy of the Selection principle to explain the.^e variation- 

 phenomena.— It is not necessary to repeat here the well-known current theoretical 

 objections to this principle, but simply to point out the bearing of this paheonto- 

 logical evidence. In Weismann's variation theory the preponderating influence 

 must be conservative, however it may explain progressive modification, or even 

 correlation of old characters, it does not admit that the genesis of new characters 

 should follow definite lines of adaptations which are not pre-existent in the germ 

 plasma. We find that new characters of the second class do follow such purpo- 

 sive or directive lines, arising simultaneously in all parts of the organism, and first 

 appearing in such minute form that we have no reason to suppose that they can 

 be acted upon by selection. The old view of nature's choice between two single 

 characters, one adaptive, the other not adaptive, must be abandoned, since the 

 latter do not exist in the second class. 



4. The most serious obstacle to the Lamarclciaii principle is the problem of trans- 

 mission. — How can peripheral influences be transmitted in the way we have out- 

 lined — now that we have such strong evidence for the continuity of the germ 



flasma? If acquired characters are not transmitted, it is clear that the whole 

 -amarckian principle is undermined, and all these instances of sequence express 

 no causal relationship. "We are then, however, left without any adequate expla- 

 nation of the laws of variations of the second class, and are thus driven to postu- 

 late some third, as yet unknown, factor in evolution to replace the Lamarckian 

 principle. 



3. Report of the Committee for improving and experimenting with a 

 Deep-sea Tow-net for opening and closing under water. — See Reports, 

 p. 111. 



