May 1900.] BOTANICAL SOCIETY OF EDINBUKGH 305 



libriiim. In the processes of cultivation, owing to the 

 way in which the pace is forced, time is not permitted 

 for each stage in the evolntion to be firmly stamped and 

 individualised in the plant registry — the chromosomes. 

 The consequences to the race as a whole are therefore 

 obscured ; but, given that, as in nature, each stage had 

 relatively considerable permanence, over - specialisation 

 would result, leading to senility and, finally, extinction. 



One can conceive of the stream of life as a continuous 

 current of low organisms, little changing, eternal ; out of 

 the common run, forms are lifted by variation to a higher 

 plane, and from this point of vantage, by further variation, 

 to successively higher and higher levels. Specialisation 

 goes on, while some die out early from unsuitable varia- 

 tions, others continue till over-specialisation accomplishes 

 their extinction. 



The main stream fiows on relatively unchanged, and 

 more forms are being picked out by variation, and all 

 working along similar — perhaps because mechanical — lines 

 produce a series of forms which, from their structural 

 resemblances, may appear as series in a continuous chain. 



Take, for example, the arborescent and herbaceous Equi- 

 setinre of the Carboniferous. Similar initial variations may 

 have started them on the upward course — the one, over- 

 specialised, became extinct comparatively early ; the other, 

 less specialised, persists to the present. 



The gigantic forms had probably attained heterospoiy, 

 and a corresponding or further stage of evolution to our 

 highest flowering plants, — a stage in which something of 

 the nature of a seed, as in these last, was formed may 

 have been attained to, and then beyond that the stag^ 

 of asexuality preceding extinction. The disappearance of 

 highly specialised types of plant life in geological times 

 is remarked, and this factor of over-specialisation may 

 have played a part in that disappearance. 



In conclusion, a reference may be made to nuclear 

 reduction, i.e. the reduction of the number of chromosomes, 

 in so far as it illustrates Mehnert's principle. 



Strasburger's (11) view, amended by Hartog (12), is 

 that it occurs in the first protistoid division after fertilisa- 

 tion, and that " it is the necessary secondary result of 



