BIOLOGICAL TRAINING AND STUDIES. 859 



it can be called, are being constantly tried in our colonies for us, 

 and on a large scale. I had taken and written here of the IBoly- 

 gonum aviculare, the 'knot' or ' cowgrass' — having learnt, on the 

 authority of Dr. Hooker and Mr. Travers (see ' Natural History 

 Review,' January, 1864, p. 134, Oct. 1864, p. 619), that it abounds 

 in New Zealand, along the roadside, just as it does in England— as 

 a glaring instance, and one which would illustrate the real value 

 of the second explanation even to an unscientific man and to an 

 unassisted eye. But on Saturday last I received by post one of 

 those evidences which make an Englishman proud in thinking that 

 whithersoever ships can float thither shall the English language, 

 English manners, and English science be carried, in the shape of 

 the second volume of the 'Transactions' of the New Zealand 

 Institute, full, like the first, from the beginning to its last page 

 of thoroughly good matter. In that volume, having looked at 

 the table of its contents, I turned to a paper by Mr. T. Kirk on 

 the 'Naturalised Plants of New Zealand,' and in this, at p. 142, 1 find 

 that Mr. T. Kirk prefers to regard the Polygonum aviculare of New 

 Zealand as indigenous in New Zealand. Hence that illustration 

 which would have been a good one falls from my hands. And 

 I must in fairness add, that because one agency is proved to be a 

 vera causa^ it is not thereby proved that no other can by any 

 possibility be competent simultaneously to produce the same effect, 

 whatever the Schoolmen with the law of Parsimony ringing in their 

 ears may have said to the contrary. I have dwelt upon this sub- 

 ject at this length with the purpose of showing how much difficulty 

 may beset the settlement of even a comparatively simple question 

 which involves only the use of the unassisted eye, or at most of 

 a simple lens. The a fortiori argument I leave you to draw for 

 yourselves, with the simple remark that the question of spontaneous 

 generation is now at least one to be decided by the microscope, and 

 by the employment of its highest powers in alliance with other ap- 

 paratus of all but equal complexity. 



We come, in the second place, to say a word as to the extent 

 of the influence which organic and living particles, of microscopic 

 minuteness but solid for all that, have been supposed, and in some 

 instances at least have been proved, to exercise upon the genesis 

 and genesiology of disease, and so upon the fortunes of our race, 

 and our means for bettering our condition, and that of our fellows. 



