J. W. Jenkinson 
451 
only gathered in with the last lot. But I have been assured by the keeper of the 
hatchery, who himself collected them, that he made no such selection ; the fish 
were taken quite at random. 
I can therefore offer no explanation. The change is certainly not to be 
attributed to any alteration in the environment. 
We pass now to the relative dimensions. These have been expressed as 
percentages of the total length. 
The means of these indices and their standard deviations are given in 
Table IV. 
First we may notice that the proportions of many organs remain constant 
throughout or nearly so — the eye, the three median fins, body-breadth, position of 
pectoral and pelvic fins. There is, however, a slight decrease in the proportion 
(except in body-breadth), which means, in the case of the position of the fins, that 
they move a little nearer the head, and this is true also of the positions of the 
median fins and of the anus. The hinder end of the operculum, on the other 
hand, moves further away from the head end : in Stage I it is in front of the 
pectoral fin, but subsequently more and more behind it. But these changes in 
the proportions of the parts are not very serious. 
It is only when we look at the length and breadth of the caudal fin that we see 
how great may be the changes that occur in their index value. The length 
increases from 4 °/ 0 to nearly 14°/ 0 °f the total body-length, the breadth from 
12°/ 0 to 18 7o (with a subsequent slight decrease). As has already been shown 
these two dimensions grow very rapidly indeed in the first period, twice or more 
than twice as rapidly as the whole body, and this excess in the growth-rate is 
maintained up to the fourth stage. By the time Stage V is reached the growth- 
rate of these parts is smaller than that of the total length, and their index value 
has also decreased. 
The figures in Table I also show that the head grows faster than the whole 
body, although in this case the excess is not so great. And again in the first two 
stages (the figures for the others are unreliable) growth-rate is high in the anterior 
dorsal and ventral fins (Table I) and the indices are correspondingly increasing. 
The variabilities (cr) of the indices are usually small. Generally speaking they 
exhibit a progressive decrease. So in the eye, head-length, length of median fins, 
breadth of body, position of paired and median fins, and of anus. In some of these 
cases (position of pelvic fin, and of median fins and of anus) there is an appreciable 
rise of variability in the final stage, though the value of the variability in the first 
stage is never reached. The two exceptions to this regular decrease of variability 
are seen in the length and breadth of the caudal fin. Here the variability first 
increases, then decreases only to rise again to more than its original value in 
passing from the fourth to the fifth stage. 
As we know already these two dimensions are anomalous in other respects. 
The mean index value rises at first rapidly then more slowly, but finally decreases 
a little ; the growth-rate is very high at first but experiences finally a sharp fall. 
57—2 
