As an example we may take the total breadth. Here the observed range of variation is from 1370 to 1227 thousandths of the standard. The unit of deviation employed is 0'004 of the carapace length ; thus, with this unit the range of deviation in the total breadth is expressed by the number 36. The crabs varied in absolute length of the standard from 200—48'6 mm., and in the fourteen groups the means are expressed in thousandths of carapace length, but the errors of mean square and all the constants that will hereafter be given are expressed in terms of the above unit of deviation.
The length of the standard is taken as a criterion of age ; this may, of course, be only roughly true, and it seems quite possible that the somewhat large fluctuations which are observed in the means are due to the groups being in reality slightly heterogeneous by including, crabs of somewhat different ages.
On glancing down the means of the several dimensions, it will be seen that a state of equilibrium is nowhere reached except perhaps in the case of the total breadth; thus, throughout life the crab is gradually changing its shape. This species, like the closely allied Garcinus moenas, is probably sexually mature when only some 20— 30 mm. long;[1] hence, when all its organs are in a rapid state of change the crab can propagate its species. Here is an ai’gument, so it would seem, against the transmission of acquired characters, for otherwise the earlier broods would tend to have a somewhat different shape to the later ones, and this is scarcely probable. To illustrate this change in shape fig. 1 was prepared. The continuous outline represents a crab with carapace length = 28‘5 mm.; the broken outline is the same crab when it has grown to 46'8 mm . ; the angle CAC' opens out by about a degree. This, of course, is only true on the supposition that natural selection has not occurred with respect to these dimensions. The error of mean square of each group is given in the columns to the right of the means. This constant we take as a measure of the variability of an organ, and in no case is there a distinct tendency for it to diminish as we pass down to the groups containing the older crabs, while in the dentary margins there is an obvious increase. Hence, at the present period in the life of the species we have no evidence of selective destruction with regard to the dimensions here discussed. It is possible, however, that all the dimensions are really more variable as the crab grows older, but that this greater variability is concealed by the action of natural selection in all Cases except in the dentary margins.
We will now treat each organ separately.
Total Breadth.—On referring to the table, it will be seen that groups 4—14 have means which remain fairly steady, and show
- ↑ This point is now being investigated. Female Carcinus occurs in berry when only some 20 mm. long.
