■ AS TO THE HEREDITY OF ACQUIRED CONDITIONS. 449 



both with regard to abnormalities by excess and many of the 

 more important abnormalities by defect. But we have now 

 arrived at a time when no thoughtful enquirer can be longer 

 content with merely cataloguiug deviations and bestowing upou 

 them sesquipedalian names such as were perhaps justifiable in 

 the days of the elder St.-Hilaire. Investigation has entered 

 upon a more strictly causal stage ; and, to my thinking, it 

 becomes evident that teratology has an important work before 

 it in relation to biological science generally, by demonstrating 

 the presence of potentialities which in the normal oi'ganism lie 

 dormant, but nevertheless must exist, or they could not in 

 exceptional circumstances show their presence." 



In the present essay my intention is to examine the recorded 

 groups of hereditary malformations, with a view to ascertain 

 their nature, so far as may be possible, whether blastogenic 

 or somatogenic. In order to do so, it will be necessary, how- 

 ever, for me to preface this portion of my paper with a short 

 account of the groups into which malformations should be causally 

 arranged. At the conclusion of the main part of the paper, that, 

 namely, which deals with hereditary malformations, I shall add a 

 few remarks on certain points in connection with the causation 

 of malformations which have not, I think, up to the present 

 received sufficient attention. 



I may perhaps here state that this paper was not under- 

 taken with a view either to support or to oppose Weismann's 

 views. My desire has been to examine the subject from aterato- 

 logical standpoint and to record the result of my observations. 



Section I. — Varieties of Malpormations. 



As has been already suggested, any etiological classification of 

 anomalies at the present time must be more or less tentative ; 

 yet I believe we may reasonably strike a line of division between 

 two great groups, each of which may be again subdivided. The 

 first group consists of malformations which are due to some error 

 in the amount of formative material, and the second of those which 

 are not. The former group maybe spoken of as non-mechanical, 

 and the latter as mechanical, provided it be understood that these 

 .terms are not used in the sense in which they are applied to the 

 ordinary occurrences of development by His, Weismann, Eoux, 

 and other writers. Errors of material may be in tw^o directions, 



linn, joukn. — zoology, vol. xxiii. 32 



