I'REFACE Vll 



Balfour's Comparative Embryology, Gadow's Amphibia and 

 Reptiles. 



The third volume, which is in the press, will deal with 

 the Tunicata, Enteropneusta, Echinodermata and Arthro- 

 poda. 



It has been pointed out to me by friends who have read 

 the proofs that I have made statements which without a 

 fuller treatment may give rise to the view that I am unortho- 

 dox on the great question of organic evolution. This is not 

 the place to give that fuller treatment, but in order to pre- 

 vent misunderstandings'! may say that any such view would 

 be erroneous. I am and always have been a convinced 

 evolutionist. I hold, that is to say, that matter is constantly 

 undergoing change, and that natural selection, taking advan- 

 age of its endless diversity in form and properties, has played 

 and is playing an important part in determining what form 

 of it (whether living or non-living) shall exist and what shall 

 cease to exist. I hold further that the forms of living matter, 

 as well as those of non-living matter, owe their existence and 

 their properties to the operation of natural laws, though 

 here we are treading on more uncertain ground, for we 

 know nothing of the origin of living matter or of the sources 

 of its properties. The chemist has made many forms of 

 matter which have, at present at least, no existence in 

 nature apart from organisms, but he has not yet succeeded 

 in making living matter. Whether he will ever be able to 

 do so is a question which may fairly be asked, but is one 

 which cannot now be answered. The view that living 

 matter arose in response to the operations of natural laws 

 cannot be either proved or disproved. It must remain a 

 matter of behef for which there is much to be said. As to 

 the origin of the manifold properties of living matter we 

 know nothing. The Darwinian theory did not account for 

 properties ; it left their origin to an imperfectly understood 

 interaction between the organism and the environment, 

 and further than this we cannot at present go. It may, 

 however, be pointed out that there are two ways in which 



