226 MAMMALIAN DESCENT. [Lect. IX. 



the bavk (iliat is just A\'liere the sap ahounds most — tlie moisture 

 which we see when wo peal a jfi-eeu witliy, exposin<^ tlie wood). 

 If this concentrically increased stem l)ecomes hollow after a time, 

 that hollo wness is due to some injur}', such as the breaking off of a 

 Ijranch, so that the wet from without drips into the wound, and tints 

 rots the inner wood. This inner wood, the " heart-wood" as it is 

 called, is dark in colour, heing filled with a dark deposit ; it has ceased 

 its vital functions, and is only kept sound hy being hermetically 

 sealed by the ncAver sap wood, or white part, and the bark outside this. 



Xow the thigh-bone of a man is as hollow as an old tree, save 

 that it is filled with marrow, the oily sul^stance that fills the cavity of 

 every long-bone. The wall of the bone is in concentric rings tir 

 tubes like the tree, and the part that is gone was in concentric rings 

 also. How did this come aljout ; where is the small bone of the 

 child which did lie where there is only a useless sort of padding now 

 — nothing but marrow 1 "We are here in the j^resence of one of the 

 jiiost marked distinctions between the animal and the plant, the !Man 

 and the Oak tree. Plants have not the power to dissolve old tissues, 

 using up the nutritious parts of a solution so made, and casting the rest 

 oif, — excreting the effete and useless matter ; Ijut animals, especially 

 the higher kinds, can do this — are always doing it ; thus every tissue 

 in the body is busy as a hive of bees. For a time, if a child or 

 youth be well fed, the deposit of new substance exceeds the removal 

 of the old, and the body increases in size throughoi;t. Leaving out 

 of consideration, for the time, the rest of the body, I may remark that 

 the organs of support undergo a most remarkal)le series of changes 

 from the time when they are difterentiated in the early embryo to the 

 time when the adult condition is attained. 



At first all is mere protoplasm ; of the endnyo, it may l)e said, 

 parts it has none, distinguishaljle in member, joint, or limb; each 

 seems either ; all ajipears to be uniform. After a time granules 

 appear, and abound; these become cells, or little living pellets of 

 protoplasm, instead of the Tuidistinguishable jelly; and, a little further 

 on, these are marshalled into ranks, and divided into groups, so that 

 we seem to have a silent sort of liuilding going on, with infinitesi- 

 mally small biicks or stones. These, liowever, are not sha])ed out- 

 side and then ]>ut together; they miulcl tlicmsclves witliin, where 

 tliey are ; then; is no sound of liaiiimer or of ax(! in the u})ri.s(.', of this 



