LECT. IX.] TIME-MARKS. 225 



might take up the embryology of the Seal, tracing the confluence of 

 the two primary germ-points, their mutual engrafting, the growth 

 of the foundations of the embryo (the " blastoderm "), then the 

 differentiation of the various tissues and organs and this second 

 piece of descriptive anatomy might serve equally well both for the 

 Seal and for Man. Then if the physiologist took up the subject, 

 the functions of every part would be found to correspond, and the 

 physiology of a Seal would be seen to be essentially the same as the 

 physiology of our own more favoured type. 



I do not ask the reader to go through all the details and experi- 

 ments for himself ; but he might waste an hour in a less pleasing 

 and profitable manner, than by comparing the Seal's skeleton with 

 that of his own species, in that model Museum in Lincoln's Inn 

 Fields. Then he would see in the skull, the spine, the chest, 

 and the limbs, part for part, joint for joint, bone for bone, the same 

 structure, but just gently altered, for somewhat different functions.; 

 altered as if by the hand not of a demon of the deep, but by a 

 kindly fairy so little difference is there in the details of the 

 skeletons of two creatures, so diverse as a Man and a Seal. 



Every organ, every part in the adult creature, Man or Seal, or indeed 

 of any complex organism whatever, has its time-marks upon it, 

 Avithin it, and throughout it. In these northern climes, where there 

 is one growing and one resting season every year, an ordinary tree 

 that has been growing for 200 years will show, when cut across, 200 

 rings of wood. But a long bone from our own body, say the thigh- 

 bone (femur), if sawn across, would show similar growth-rings to 

 those seen in the tree when sawn across. Of course, as \ve are 

 not fixed trees, but active men, not standing as if we could not 

 find our hands, but supplying ourselves Avith food constantly, our 

 growth-rings do not depend upon the seasons, though they are the signs 

 of the lapse of time. Now, between the growth of the tree and that 

 of the thigh-bone, there is a remarkable similarity and an equally 

 remarkable difference. The soft tissues of the embryo of a tree, the 

 oak, for instance, which gro\v into the axis or rudimentary stem, 

 gradually become differentiated into various kinds of cells, which 

 form the pith, medullary rings, large ducts, woody fibres, and the 

 inner and outer layers of the bark. Year by year an additional ring, 

 or tube of wood, is formed outside the last year's growth, and inside 



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