180 DR. P. CHALMERS MITCHELL ON THE 



length is at first surprising, as we are accustomed to regard fish as being readily digestihle. 

 However, a bird does not eat fish carefully with a knife and fork, but bolts it whole. 

 Dr. Hutchison, a recent writer on foods (17), calls attention to the large amount of 

 waste matter in uncooked fish, amounting to fully seventy per cent. 



Third Homoplastic Modification. Shortening of the gut in frugivorous birds. The 

 tendency of the gut in birds and mammals which live chiefly on fruit to be very short, 

 thin-walled, and wide is well known, and is in direct relation to two simple physiological 

 factors. The nutritious substances in fruits are in a form which renders them capable 

 of rapid and fairly complete absorption, and the organic salts present stimulate osmosis. 

 The ease of absorption makes a relatively large surface unnecessary, and the large calibre 

 of the gut not only diminishes the outflow from the blood caused by the presence of 

 organic salts, but it decreases the danger of violent purging. The vigour of peristalsis 

 in birds is remarkable ; when the duodenum of a pigeon contracts it becomes as hard 

 and tense as a piece of cartilage. 



Fourth Homoplastic Modification. Increase of length rather than of calibre in large 

 birds. I am not aware that the circumstance has been noticed, but it is the case that 

 in large birds and large mammals the gut tends to be relatively longer than in small 

 birds and small mammals. The explanation, I think, is simple. In the course of 

 phylogenetic increase in size, the various organs increase in size correlatively with the 

 whole, but in a mode corresponding to their functions. The first business of the. 

 intestine is to present to the food-absorbing surface sufficient to supply the needs of the 

 whole body. As the calibre of a tube increases, its capacity increases more rapidly than 

 its surface ; it follows that to preserve the same relation of intestinal surface to intestinal 

 capacity, the length of the gut must increase more than the calibre in the course of 

 phylogenetic increase of size. It is interesting to notice that the caeca and the rectum, 

 two portions of the gut of birds in which absorption is not so great, increase in large 

 birds almost as much in calibre as in length, so that very frequently large birds display 

 CEeca and rectum which appear to be much wider than the anterior portions of the 

 intestinal tract. It is plain, from what has been said, that no genetic significance is to 

 be attached to such conditions, which are merely a result of the homoplastic modifi- 

 cation due to large size of the whole creature. 



There is no need to discuss here the difficult problems as to the mode of origin of 

 homoplastic resemblances. To some extent they may be freshly epigenetic in each 

 generation; and Gadow's (12) investigations into the lengths of the gut in chicks as 

 compared with adults would seem to show that there is much to be learned as to the 

 occurrence of changes of form and length in direct relation to changes of diet. They 

 may have come about by a slow selection of strains with genetic variations in the 

 direction of increased length or of capacity to acquire increased length at the stimulation 

 of food; or, on neo-Larnarckian principles, they may be the summations of the effect of 

 stimulations in a series of generations. It is enough to state that these homoplastic 

 modifications must be allowed for or " corrected " before the drawing of conclusions as 

 to relationship. Before the condition of an intestinal tract can be taken as affording a 

 clue to the affinities of its possessor, it must, in imagination, be shortened, in the case 



