278 Mr. H. G. Seeley on the Origin 



attains enormous width from side to side. The animal's me- 

 thod of burrowing causes a great use of the pectoral muscles ; 

 and the use of these muscles coincides with the condition of 

 their attachment for the development of a sternum similar in 

 form to that of a bird ; and true coracoid bones are attached to 

 it, as in birds. In quadruped animals which carry the head 

 and neck erect, like the giraffe, where the vertebrae experience 

 the weight of the head and part of the neck above each in 

 pressure, made intermittent by activity, the vertebree are 

 found to attain enormous length ; but the upper bones are the 

 longer ones : whence it may be inferred that a moderate inter- 

 mittent pressure is more favourable to growth than a considera- 

 ble pressure, the greater pressure producing what is relatively 

 atrophy ; and in the elephant, where tlie pressure of the head 

 upon the vertebrae is great, and not greatly varied, the force of 

 growth is unable to overcome the weight, and the vertebraj are 

 short from back to front, though they grow at their circum- 

 ference. The same shortening of the neclc-vertebrge, connected 

 with the continuous pressure of a large head, is admirably seen 

 in the Cetacea, where in progression the neck-vertebra have to 

 support the non-intermittent pressure of the immense head. 

 Whatever and wherever the pressure and tension are mani- 

 fested, it is always with this result in increase or decrease of 

 growth, which vary as the pressure is intermittent or constant. 

 Examples of it could only cease when the enumeration of 

 organisms was terminated. 



But the inference from these facts is not merely that the 

 same law holds true for growth in the different parts of the 

 skeleton and in the whole skeleton as governs the growth of 

 a single bone and of its parts, but that the whole distinctive 

 plan of the part which is inherited from individual to indivi- 

 dual is as completely in harmony with this law of growth as 

 though it had been produced not by inheritance at all, but 

 wholly by mechanical causation, in the individual animal in 

 which it is visible ; that is, growth in the individual and 

 growth in the plan of the individual are commonly in the 

 same directions, and such as would have been produced by 

 the continuous action of the same cause, namely intermittent 

 pressure and tension. But it is seen that only an infinitesimal 

 element of the plan of the animal is produced by the indivi- 

 dual ; hence, since the plan-growth exists in all the individuals 

 of a group, it is justly inferred that the plan has accumulated 

 in the sum of the individuals by being passed on from gene- 

 ration to generation ; for in that way, and in that way only, 

 could the mechanical law act which has been seen to have 

 acted in the daily life of animals, so as to produce the forms 



