549 
all forms of fishes were the eggs and milt, or rather the 
micropyle and the spermatozoa, the same size in all, the specific 
gravity similar, and the period of breeding identical, it is these 
very differences which act as a bar to hybridization. For a 
mechanical difficulty * is in existence impeding, or in some 
instances preventing, the eggs of small forms or small fishes, 
even when of the same species, being fertilized by larger 
parents; while the seasons for breeding are likewise a great 
bar, for should two forms not be doing so at the same period of 
the year, it is evident they could not cross. If all forms could 
interbreed freely one with another, we might find minnows and 
salmon, perches and bullheads, sticklebacks and carps hybridiz- 
ing, and in a comparatively short space of time families, genera 
and species would be things of the past. Were this to occur 
the result would be readily foretold: at present small species 
obtain sustenance in small as well as in large pieces of water, 
but were the smaller forms to merge into the larger, our brooks 
or lesser streams and smaller ponds would be no longer stocked 
with fish suitable to the size of their waters, as the amount of 
food would be insufficient to maintain them in health, even 
could it sustain their lives. But this is a question I need not 
follow further at present. 
On February 1st, 1887, I exhibited, at a meeting of 
the Zoological Society, an example of a hybrid between the 
“ Eegs of a small brook trout obtained from streams it is almost im- 
possible to fertilize with salmon milt from large fish, but if a salmon smolt 
or grilse is employed the difficulty ceases. For on November 29th, 1883, 
4,500 eggs of a fine Lochleven trout were thus milted at Howietoun, 
and the loss during incubation was only 6 per cent.; while on December 
27th, 1884, 7,000 eggs of a large Lochleven trout were milted from an adult 
salmon with the loss of 28 per cent. The following sizes of eggs measured 
at Howietoun, show how they vary withage :—Diameter of grilse eggs, from 
0°20 to 0-22 of an inch; of a 16 lb. salmon, 0°24 inch; larger fish, 0°25 inch 
to 0°30 inch. Lochleven trout, 2 and 3 years old, 0°17 inch; 6 years old, 
0:18 to 0°19 inch ; 8 years old, ():20 to 0°24 inch. American char, 2-year-old, 
(0.14 inch ; 3-year-old, 0°17 inch ; 4-year-old, 0°18 inch. If therefore the 
size of the eggs increase along with the increased size and age of the parent, 
it is most likely that spermatozoa similarly augment, 
