APRIL 25, 1884.] 
increase or diminution of one results in the in- 
crease or diminution ofall. Ifthe agencies of 
the first order —i.e., secular cooling, heating 
of the sun, and astronomic stresses — be neg- 
lected, the other agencies are interdependent 
in such a manner that there is a tendency secu- 
larly to establish an equilibrium; and doubt- 
less such an equilibrium would be established 
in a period not of great length considered 
geologically. But the agencies of the first 
order continuously destroy the static equilib- 
rium, and, conjoined with the-others, they pro- 
duce the sequence of changes discovered in 
geologic history. 
The rate of internal cooling is manifestly 
diminishing, and physicists incline to the opin- 
ion that the heating due to the sun is diminish- 
ing. From this stand- point, then, the rate of 
change in geologic history is secularly dimin- 
ishing. On the other hand, the secondary 
agencies of change increase in efficiency by 
reason of increased heterogeneity in the struc- 
ture of the crust. From the irregularities of 
the upper surface, and those probably existing 
at the lower, as suggested by many facts, the 
crust is heterogeneous i in thickness, and doubt- 
less is becoming more so. It also becomes 
more and more heterogeneous in constitution 
by the progressing differentiation of its parts, 
exhibited in the diversification of geologic for- 
mations, density, temperature, conductivity, hy- 
dration, and chemical and lithical constitution. 
This internal heterogeneity renders the crust 
more sensitive to external agencies of change, so 
that a smaller amount of primary change serves 
to unlock a given amount of secondary change. 
At the present stage of geologic research the 
facts are not sufficient to establish the quan- 
titative relation between the diminished rate 
of change from the primary agencies and the 
increased rate of change from the secondary 
agencies. It is therefore impossible to predi- 
cate any variation in the rate of change from 
the close of archaean time to the present. 
J. W. PoweELt. 
EVOLUTION OF THE DECAPOD ZOEA. 
PRINCIPLES applicable to adults are often equally 
applicable to larvae. In the discussion of natural se- 
lection most writers have confined themselves to adult 
animals and their reaction upon environment. There 
is no reason, however, why the principle should not 
be extended to include larval forms; and, indeed, toa 
slight extent this has already beendone. Weismann’s 
‘Theory of descent’ proceeds upon this line, and in- 
dicates some of the important results which may arise 
from such research. Crustacean larvae offer particu- 
SCIENCE. 
513 
larly good opportunity for work in this direction- 
They are abundant, are easily obtained, and readily 
studied. ‘They present great varieties of form, which 
are frequently not in any degree related to the adult 
characteristics. Indeed, crustacean larvae seem al- 
most like a distinct group of animals, and may be 
studied as such, with the extra advantage that they 
are highly variable, and undergo rapid metamorpho- 
sis. Some of the possibilities of such research may 
be seen by a short consideration of the different forms 
of decapod zoea. 
To make the subject clear, it will be necessary to 
give a brief description of three types of decapod 
larvae, confining ourselves, however, only to such 
points as particularly concern us here. ‘The first is 
the type, which is undoubtedly the oldest, known as 
the protozoea. It is acomparatively rare form, being 
found in a few macruran species (Peneus, Lucifer, 
Euphausia). Fig. 1 represents such a larva. As far 
as concerns us, the peculiarities are these: the long 
body consists of a large cephalothorax, a more or less 
complete thorax, and an abdomen. The important 
point is, that all of the regions of the body are repre- 
sented. When viewed from above, the part of the 
body composed of thorax and abdomen is seen to be 
very slender and weak, and to extend for a long dis- 
tance backwards. A second important point is the 
method of locomotion: unlike all other forms, the 
antennae, instead of being sensory organs, are used in 
locomotion. They are large, and covered with swim- 
ming-hairs, which convert them into paddles; and, by 
moving them to and fro, the protozoea slowly propels 
itself by a series of jerks through the water. The 
telson is a third important feature: it is small, being 
in our figure no broader than the abdomen; it is 
usually forked, and carries a number of long spines 
(typically seven, though the number varies); it is not 
a swimming-organ, —a point of particular interest. 
One other feature must be mentioned, —the usual 
though not universal absence of protective spines. 
A second type is that of the ordinary macruran 
zoea; e.g., the larva of the common shrimp. Sucha 
zoea is represented in fig. 2. Here we find a number 
of changes. First we see that only two'regions of the 
body are present, the cephalothorax and the abdomen, 
the thorax being unrepresented. The cephalothorax 
is not very different from that of the protozoea. The 
abdomen is, however, very different: it is distinctly 
divided into segments, all of which are well developed ; 
it is tolerably thick, and is a much more powerful 
structure than the corresponding part of the proto- 
zoea. ‘The muscular and usually the nervous system 
is well developed. In short, the abdomen is much 
more perfect than that of fig. 1. The locomotion of 
this zoea is entirely different from that of the proto- 
zoea. It does not use its antennae for moving, but 
propels itself vigorously with powerful strokes of its 
abdomen, after the manner of the lobster: at least, 
this is its motion when trying to escape danger; and 
that is all that concerns us. In correlation with this 
changed locomotion, the antennae have altered their 
form, and are now true sense-organs. On the other 
hand, the telson has become broadened into a flat 
