8 SCOTT. [Vol. V. 



these cases we are not restricted to a study of fragments and 

 conjectural restorations, but of many genera we have nearly 

 complete skeletons and satisfactory information as to geograph- 

 ical and geological distribution. Of course, our knowledge of 

 these phyla is very far from complete, but it is sufficiently so to 

 promise very important aids to the solution of the evolutionary 

 problems which have been enumerated. 



Aside from the questions as to the mode in which develop- 

 ment operates, there are others of more fundamental importance ; 

 viz. those as to the efficient causes or factors of development 

 and the laws of heredity. The essays of Weismann have of late 

 aroused widespread discussion as to these factors, and have 

 thrown grave doubts upon the transmission of acquired charac- 

 ters, which we have hitherto accepted as a fundamental axiom. 

 Weismann's objections to assuming such transmission are two- 

 fold : (i) the impossibility of imagining how changes in the 

 adult organism can so affect the germ-plasm as to cause corre- 

 sponding changes in the offspring ; and (2) the entire absence of 

 proof that characters so acquired ever are transmitted. Of 

 course neither of these reasons can be conclusive, as Weismann 

 unhesitatingly admits. " It is perfectly right to defer all expla- 

 nation and to hesitate before we declare a supposed phenomenon 

 to be impossible, because we are unable to refer it to any of the 

 known forces. No one can believe 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 accept- 

 ance is unavoidable" (No. 33, pp. 80, 81). "It appeared to me 

 to be necessary to state this weighty and fundamental question, 

 and to formulate it concisely and definitely, for only in this way 

 will it be possible to arrive at a true and lasting solution of the 

 problem. We must, however, be clear on this point — that the 

 understanding of the phenomena of heredity is only possible on 

 the fundamental supposition of the continuity of the germ-plasm. 

 The value of experiment in relation to this question is somewhat 

 doubtful. A careful collection and arrangement of facts is far 

 more likely to decide whether, and to what extent, the continuity 

 of germ-plasm is reconcilable with the assumption of the trans- 

 mission of acquired characters from the parent body to the germ, 



