226 MAMMALIAN DESCENT. [Lect. IX. 



the bark (that is just where the sap abounds most — the moisture 

 which we see when we peal a green withy, exposing the wood). 

 If this concentrically increased stem becomes hoUow after a time, 

 that hollo wness is due to some injury, such as the breaking off of a 

 branch, so that the wet from without drips into the wound, and thus 

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

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

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

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



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

 that it is filled with marrow, the oily substance that fills the cavity of 

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

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

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

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

 — nothing but marrow ? We are here in the presence of one of the 

 most 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 

 off, — excreting the effete and useless matter ; but 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 throughout. Leaving out 

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

 the organs of support undergo a most remarkable series of changes 

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

 time when the adult condition is attained. 



At first all is mere protoplasm ; of the embryo, it may be said, 

 parts it has none, distinguishable in member, joint, or limb; «ach 

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

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

 protoplasm, instead of the undistinguishable 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 building going on, with infinitesi- 

 mally small bricks or stones. These, however, are not shajjed out- 

 side and then put together ; they model themselves within, where 

 they are ; there is no sound of hammer or of axe in the uprise of this 



