No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 379 



too much for granted, and that the foundations of one essential 

 part of current evolutionary belief were insecurely laid. The 

 whole subject needs, therefore, to be exhaustively re-examined 

 and tested from every side. 



For the purposes of this paper it is not necessary for me to 

 enter into the reasons of my dissent from Weismann's theory 

 of the continuity of the germ-plasm, nor to explain why, in my 

 opinion, so far from rendering the phenomena of heredity 

 more intelligible, it tends to confuse them still further and to 

 end logically in a system very like the old preformationism. 

 As Lloyd Morgan has very pithily put it, " I cannot but regard 

 Weismann's doctrine of the continuity of germ-plasm as a dis- 

 tinctly retrograde step. His germ-plasm is an unknowable, in- 

 visible, hypothetical entity. Material though it be, it is of no 

 more practical value than a mysterious and mythical germinal 

 principle. By a little skilful manipulation it may be made to 

 account for anything and everything. The fundamental as- 

 sumption that whereas germ-plasm can give rise to body-plasm 

 to any extent, body-plasm can under no circumstances give rise 

 to germ-plasm, introduces an unnecessary mystery. Biological 

 science should set its face against such mysteries " (No. 39, pp. 

 141 and 142). But into this subject we need not enter, because 

 so far as the mammals are concerned, the difficulty of compre- 

 hending how somatic changes can affect the germ in appropriate 

 fashion so as to reproduce these changes in the offspring, is in 

 no way diminished by a denial, or even a complete refutation of 

 the continuity theory. Weismann states the difficulty thus : " It 

 is perfectly right to defer all explanation, and to hesitate before 

 we declare a supposed phenomenon impossible, because we are 

 unable to refer it to any of the known forces. No one can be- 

 lieve that we are acquainted with all the forces of nature. But, 

 on the other hand, we must use the greatest caution in dealing 

 with unknown forces, and clear and indubitable facts must be 

 brought forward to prove that the supposed phenomena have a 

 real existence, and that their acceptance is unavoidable" (No. 57, 

 pp. 80 and 81). But does the assumption that acquired charac- 

 ters are transmissible involve, as a matter of fact, an appeal to 

 unknown forces ? Rather, I should say, to the as yet not well 

 understood operation of known forces. In another connection, 

 Weismann has himself well defended this principle, when, in 



