I presume that lowness here means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should not have preserved or rejected each little deviation of form so carefully as when the part has to serve for some one special purpose.

In the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular purpose must be of some particular shape.

Natural selection, it should never be forgotten, can act solely through and for the advantage of each being.
Rudimentary parts, as is generally admitted, are apt to be highly variable.

We shall have to recur to this subject; and I will here only add that their variability seems to result from their uselessness, and consequently from natural selection having had no power to check deviations in their structure.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER, IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY VARIABLE.
Several years ago I was much struck by a remark to the above effect made by Mr.Waterhouse.Professor Owen, also, seems to have come to a nearly similar conclusion.

It is hopeless to attempt to convince any one of the truth of the above proposition without giving the long array of facts which I have collected, and which cannot possibly be here introduced.