CAUSES OF SCIENTIFIC KNOWLEDGE 287

As little is it conceivable that they should have been brought

about by " Natural Selection," as that it should have caused

the pearly lining of shells or their sub-superficial beauty, or

that of gems and other minerals buried for ages in the bowels

of the earth.

One of the most obvious characters presented by the

bodies of animals, including our o\vn, is that each has a right

and left side, and that these two sides, and their parts,

correspond as our right hand proverbially resembles our

left one. When deeply considered, this fact is by itself

sufficient to prove that the body of an animal has its own

innate laws, which regulate its development ; for this kind of

correspondence technically called "bilateral symmetry"

shows itself not only in these familiar conditions, but in the

effects of disease and in very peculiar structures found in

some exceptional species of animals. Indeed, on the

hypothesis that a blood-relationship of descent binds to-

gether different kinds of animals, nature actually forces

upon us the perception that new and more intensely marked

forms of bilateral symmetry have arisen in a space of time

which, geologically considered, must be called brief. Thus,

naturalists now are generally agreed that birds have

descended from reptiles ; but the very diversity of the

bilateral symmetry which exists between the two wings of

birds on the one part, and between their two legs on the

other part, is far more striking than any which is found in

their hypothetical progenitors.

Another form of bodily symmetry in animals is known as

"serial symmetry." Such symmetry is most plainly seen

and obvious in the successively similar segments and pairs

of limbs in the centipede and its allies ; but it is also to

be traced in the bony structure of the human chest, with

its successive ribs, in the series of bones (called vertebrae)

which compose our spinal column or backbone, and in the

As little is it conceivable that they should have been brought

about by " Natural Selection," as that it should have caused

the pearly lining of shells or their sub-superficial beauty, or

that of gems and other minerals buried for ages in the bowels

of the earth.

One of the most obvious characters presented by the

bodies of animals, including our o\vn, is that each has a right

and left side, and that these two sides, and their parts,

correspond as our right hand proverbially resembles our

left one. When deeply considered, this fact is by itself

sufficient to prove that the body of an animal has its own

innate laws, which regulate its development ; for this kind of

correspondence technically called "bilateral symmetry"

shows itself not only in these familiar conditions, but in the

effects of disease and in very peculiar structures found in

some exceptional species of animals. Indeed, on the

hypothesis that a blood-relationship of descent binds to-

gether different kinds of animals, nature actually forces

upon us the perception that new and more intensely marked

forms of bilateral symmetry have arisen in a space of time

which, geologically considered, must be called brief. Thus,

naturalists now are generally agreed that birds have

descended from reptiles ; but the very diversity of the

bilateral symmetry which exists between the two wings of

birds on the one part, and between their two legs on the

other part, is far more striking than any which is found in

their hypothetical progenitors.

Another form of bodily symmetry in animals is known as

"serial symmetry." Such symmetry is most plainly seen

and obvious in the successively similar segments and pairs

of limbs in the centipede and its allies ; but it is also to

be traced in the bony structure of the human chest, with

its successive ribs, in the series of bones (called vertebrae)

which compose our spinal column or backbone, and in the