946 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol. VIII. 
Another of the principal of these factors is the effort for preservation 
of the type—in other words, the desire to win a mate and generate 
offspring amid a crowd of competitors of the same species. If this 
statement of the case seems somewhat unromantic, let me bring before 
you some extreme illustrations in its .support. Here is a specimen of 
a peculiar crustacean known as Sacculina, which, as you see, is a shapeless 
mass of tissue firmly fixed to the tail of this crab, In its early larval stages, 
Sacculina resembles not perhaps the crab, since the crab’s earlier larval stages 
are somewhat different from those of other crustacea, but the prawn; so that 
it has—for a crustacean—noble relatives and a high ancestry. Both Sacculina 
and the prawn have at first the form of small free-swimming larve with a pair 
of antennz, two pairs of biramous legs, a mouth, a single median eye, &c., 
known as Nauplius. Now the necessity for self-preservation has led Sacculina 
to attach itself as a parasite to some other animal. Being a parasite and 
entirely dependent on this other animal, or host, it no longer requires 
organs of sense to see its prey or, escape its enemies ; it no longer wants legs 
to swim about in search of prey, and its existence being assured as a parasite, 
it loses all resemblance to its nobler relatives and becomes nothing but a large 
stomach encapsuled, along with its reproductive organs, in a bag of skin, 
- Sacculina then exhibits in an extreme degree the truth of the proposition, that 
the form of an animal is very very largely determined by the manner in which 
it gets its living. 
Again, let this Gelasimus crab be a witness of the truth of the fact, that the 
aim of preserving the type—in other words of winning a mate and leaving 
offspring—has a great effect in shaping an animal. Here are two specimens of 
Gelasimus annulipes—a male and a female. Now the females of this Gelasimus 
are relatively scarce, while the males are relatively very numerous, so that it 
is a difficult thing for the male Gelasimus to win a wife. To obtain a wife he 
must fight with his fellows, and this necessity of existence has led to the 
wonderful development of one of his chelipeds asa fighting weapon—a develop- 
ment so extraordinary, as to make the animalarather ridiculous appendage. 
to his own claw. That this claw is not otherwise necessary for existence is 
proved by the fact that the female does not possess it, 
In a nexus of animal life the necessity to get a living—i. ¢., the necessity to 
eat—involves and implies a necessity to escape being eaten, and this necessity 
to avoid falling a victim becomes a third factor of great importance in deter- 
mining the forms of animals. Asan extreme illustration of the influence of 
this third factor, let me show you this beautiful specimen of Oreophorus—a 
small, timid, inoffensive crab which, for the purpose of concealment from its 
enemies, has become so modified in form as to resemble a piece of eroded 
coral shingle, rather than an animal. 
To recapitulate then, we find one, or all, of four principal factors at work in 
determining the peculiarities of form of animals—viz., (1) peculiarities in the 
