ed for sewing were knotted at the ends.[98] It is
impossible not to admire animals who have skilfully triumphed over all
the obstacles met with in the course of these complicated
operations.[99]

[98] _Catalogue of Birds, etc._, p. 16.

[99] Tristram, "On the Ornithology of Northern Africa," _Isis_, 1859-60.

[Illustration: FIG. 31.]

Certain Spiders, while they do not actually sew in the sense that they
perforate the leaves they use to build their nest, and draw the thread
through them, yet subject the leaves to an operation which cannot well
be called anything else but sewing it.[100]

[100] McCook describes, and gives good illustrations of,
these nests in various stages of progress, _American
Spiders_, vol i. p. 302.

_Modifications of dwellings according to season and climate._--A
certain number of facts show that these various industries are not
fixed and immutable instincts imposed on the species. Certain Birds
change the form of their dwelling according to the climate, or
according to the season in which they inhabit it. For example, the
Crossbill, _Loxia taenioptera_ (Fig. 32), does not build its nest
according to the same rules in Sweden as in France. It builds in every
season. The winter shelter is spherical, constructed with very dry
lichens, and it is very large. A very narrow opening, just sufficient
for the passage of the owner, prevents the external cold from
penetrating within. The summer nests are much smaller, in consequence
of a reduction in the thickness of the walls. There is no longer need
to fear that the cold will come through them, and the animal gives
itself no superfluous trouble.

[Illustration: FIG. 32.]

Again, the Baltimore Oriole, which inhabits both the Northern and
Southern States of North America, knows very well how to adapt his
manner of work to the external circumstances in which he lives. Thus,
in the Southern States the nest is woven of delicate materials united
in a rather loose fashion, so that the air can circulate freely and
keep the interior fresh; it is lined with no warm substance, and the
entrance is turned to the west so that the sun only sends into it the
oblique evening rays. In the north, on the contrary, the nest is
oriented to the south to profit by all the warm sunshine; the walls
are thick, without interstices, and the dwelling is carpeted in the
warmest and softest manner. Even in the same region there is great
diversity in the s