Silk and its Secrdors. 
[June, 
348 
But silk-spinning extends into the world of waters. The 
Pinna and several mussels have a silk-gland and attach their 
shells to stones, &c., by a number of threads. It is needless 
to say that in this case the secreting and spinning apparatus 
is essentially different from that which we meet with in 
inserts and spiders. 
But having traced the production and use of silk so far 
there comes, perhaps, the hardest question of all : Why 
does it extend no further ? Why do we find no vertebrate 
animals capable of spinning ? It would be too hazardous 
to say that such a power, if it existed, would be of no use 
to such animals. If we run over in our minds the various 
purposes to which silk is applied by the Arthropods, we 
must admit that, if certain mammals, birds, or reptiles 
could spin, it would often give them a certain advantage in 
the struggle for existence. Suppose that a bird or beast of 
prey could construct a web between trees or in thickets, 
would not its prospeCtts of securing a fair supply of booty 
be very distinctly increased ? Suppose that a bird when 
building its nest had the power of tieing its materials 
together and mooring them to the branches of trees with 
silk cords, would not its home be more secure ? There is 
however, a reason to the contrary ; a net large and strong 
enough to support, say, an animal of the size and weight 
of a fox or of a wild cat and in addition the hares, rabbits, 
birds, &c., upon which these destroyers prey, would have 
to be very strong, and would require a very large quantity 
of silk for its construction. Now, as we have seen, so highly 
nitrogenous a secretion as silk can only be elaborated at the 
outlay of an excessive quantity of food. It may well be, 
therefore, that the process would, figuratively speaking, not 
prove remunerative. In other words, the additional quantity 
of food captured by the aid of the net might not compensate 
for the additional quantity expended in the secretion of the 
silk. 
Or, again, it may be that in the vertebrates the power 
of secreting varied products, colouring-matters, odours, 
poisons, and also silk is relatively feebler than among the 
Arthropods. 
