Chap. XXVII. OF PANGENESIS. 389 



through widely different forms; or conversely, after passing 

 through nearly the same early forms, arrive at a widely 

 different termination. In these cases it is very difficult to believe 

 that the early cells or units possess the inherent power, inde- 

 pendently of any external agent, of producing new structures 

 wholly different in form, position, and function. But these cases 

 become plain on the hypothesis of pangenesis. The organic 

 units, during each stage of development, throw off gemmules 

 which, multiplying, are transmitted to the offspring. In the 

 offspring, as soon as any particular cell or unit in the proper 

 order of development becomes partially developed, it unites with 

 (or to speak metaphorically is fertilised by) the gemmule of the 

 next succeeding cell, and so onwards. Now, supposing that at any 

 stage of development, certain cells or aggregates of cells had been 

 slightly modified by the action of some disturbing cause, the cast- 

 off gemmules or atoms of the cell-contents could hardly fail to 

 be similarly affected, and consequently would reproduce the same 

 modification. This process might be repeated until the struc- 

 ture of the part at this particular stage of development became 

 greatly changed, but this would not necessarily affect other 

 parts whether previously or subsequently developed. In this 

 manner we can understand the remarkable independence of 

 structure in the successive metamorphoses, and especially in the 

 successive metageneses of many animals. 



The term growth ought strictly to be confined to mere 

 increase of size, and development to change of structure. 43 Now, 

 a child is said to grow into a man, and a foal into a horse, but, 

 as in these cases there is much change of structure, the process 

 properly belongs to the order of development. We have indirect 

 evidence of this in many variations and diseases supervening 

 during so-called growth at a particular period, and being in- 

 herited at a corresponding period. In the case, however, of 

 diseases which supervene during old age, subsequently to the 

 ordinary period of procreation, and which nevertheless are some- 

 times inherited, as occurs with brain and heart complaints, we 



^Various physiologists have insisted instance in microcephalous idiots, in 



on fhu distinction between growth and which the brain continues to grow 



development Prof. Marshall (< Phil. after having been arrested in its deve- 



Iransact., 1864, p. 544) gives a good lopment 



