LARVA AND EMBRYO. 47 
The larva is an immature organism functionally adapted 
for its external environment at every stage. Very often the 
larva passes through a succession of environments before 
becoming adult, and the series is known as the ontogenetic 
migration of the.species. For example, a cod is a ground- 
feeder and lives at moderate depths near the sea-bottom, but 
the egg and larva are pelagic, living in the surface-water of 
the open sea. The larva migrates inshore to the shallows 
before moving out to join its fellows, thus performing an 
ontogenetic migration. At each stage its structure is adapted 
for its particular environment ; whilst pelagic it is transpar- 
ent, when inshore its coloration helps to hide it, and so on. 
In the second, or Embryonic type, the developing ovum 
is supplied with nourishment, in one form or another, from 
the parent and is protected from the outside world by a 
shell, or by the body of the parent, until all its earliest stages 
are passed, when it leaves its protecting envelope more 
or less like its parent. 
In the ideal embryonic type cell-formation is completed 
before differentiation commences, a condition nearly attained 
in the embryo of some vertebrates. Asa general rule, the 
lower and more primitive members of a marine phylum 
develop by the larval method and the higher members of 
marine phyla, together with nearly all terrestrial forms, have 
an embryonic development. 
The past descent of a group of animals is known as phylogeny, and 
in nearly every known instance this past descent reveals a long change 
of environment of the successive generations, or phylogenetic migration. 
Thus it is usually held that our land amphibians, like the frog, are 
descended from aquatic ancestors which must have gradually, as time 
went on, migrated from the sea to fresh water and from fresh water 
to marsh and eventually to dry land. Doubtless these ancestors were 
fish-like in their characters at the epoch when they lived in the sea 
and the rivers, but they gradually acquired amphibian characters as the 
dry land was reached. 
If we picture to ourselves the succession of individuals in this 
instance we see that each must have passed through the same stage of 
structure as its predecessor and then passed a little further on. Thus 
the individual A was a fish and lived an aquatic existence. The in- 
dividual B, its progeny, lives in the same surroundings, and by the 
primary law of heredity he develops like his parent, but as he has taken 
to slightly more air-breathing habits his structure adapts itself slightly 
to this change of environment and traces of amphibian characters begin. 
His progeny C will tend to resemble his parent and will pass through 
the fish-structure of A to the partially amphibian structure of B. Hence 
