702 ADDRESS TO THE DEPARTMENT OF ANATOMY 



Before the appearance of the Origin of Species many very 

 valuable embryological investigations were made, but the facts 

 discovered were to their authors merely so many ultimate facts, 

 which admitted of being classified, but could not be explained. 

 No explanation could be offered of why it is that animals, in- 

 stead of developing in a simple and straightforward way, un- 

 dergo in the course of their growth a series of complicated 

 changes, during which they often acquire organs which have no 

 function, and which, after remaining visible for a short time, dis- 

 appear without leaving a trace. 



No explanation, for instance, could be offered of why it is 

 that a frog in the course of its growth has a stage in which it 

 breathes like a fish, and then why it is like a newt with a long 

 tail, which gradually becomes absorbed, and finally disappears. 

 To the Darwinian the explanation of such facts is obvious. The 

 stage when the tadpole breathes by gills is a repetition of the 

 stage when the ancestors of the frog had not advanced in the 

 scale of development beyond a fish, while the newt-like stage 

 implies that the ancestors of the frog were at one time organized 

 very much like the newts of to-day. The explanation of such 

 facts has opened out to the embryologist quite a new series of 

 problems. These problems may be divided into two main 

 groups, technically known as those of phylogeny and those of 

 organogeny. The problems of phylogeny deal with the ge- 

 nealogy of the animal kingdom. A complete genealogy would 

 form what is known as a natural classification. To attempt to 

 form such a classification has long been the aim of a large 

 number of naturalists, and it has frequently been attempted 

 without the aid of embryology. The statements made in the 

 earlier part of my address clearly shew how great an assistance 

 embryology is capable of giving in phylogeny ; and as a matter 

 of fact embryology has been during the last few years very 

 widely employed in all phylogenetic questions, and the results 

 which have been arrived at have in many cases been very 

 striking. To deal with these results in detail would lead me 

 into too technical a department of my subject ; but I may point 

 out that amongst the more striking of the results obtained 

 entirely by embryological methods is the demonstration that the 

 Vertebrata are not, as was nearly universally believed by older 



