finds that _section for section_
the organ in the skate is as efficient as it is in _Torpedo_. It is
evident that these facts also point to the skate's organ being in course
of phylogenetic evolution.

[Illustration: Fig. 118.--_Raia radiata_, representing the life size
of the youngest individual in which muscle fibres have been found
developing into electric cells].

[Illustration: Fig. 119.--Electric organ of the skate. The left-hand
drawing (i) represents the entire organ (natural size) of a
full-grown _r. radiata_. This is a small skate, which rarely exceeds
50 centms. in length; but in the large _r. batis_, the organ may
exceed two feet in length. The other drawings represent single
muscle-fibres in successive stages of transition. In the first of
the series (ii) the motor plate, and the nerves connected with it,
have already been considerably enlarged. In the other three
specimens, the fibre becomes more and more club-like, and eventually
cup-like. These changes of shape are expressive of great changes of
structure, as may be seen in the last of the series (v), where the
shallow cup is seen in partial section. The electric plate lines the
concavity of the cup, and is richly supplied with nerves (only a few
of which are represented in the last drawing); the thick walls of
the cup are composed of muscular fibres, the striation of which is
distinctly visible.]

[Illustration: Fig. 120.--Electric cells of _raia radiata_. The
drawing on the left represents one of the clubs magnified, as in the
preceding wood-cut. The drawing on the right represents a number of
these clubs, less highly magnified, _in situ_.]

Again, it cannot be answered that the principle of correlation may be
drawn upon in mitigation of the difficulty. The structure of the
electric organ is far too elaborate, far too specialized, and far too
obviously directed to a particular end, to admit of our conceivably
supposing it due to any accidental correlation with structural changes
going on elsewhere. Even as regards the initial changes of
muscle-elements into electrical-elements, I do not think the principle
of correlation can be reasonably adduced by way of explanation; for, as
shown in the illustrations, even this initial change is most
extraordinarily peculiar, elaborate, and specialized. But, be this as it
may, I am perfectly certain that the principle of