336 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
of varieties have in each case been produced from a single original 
stock. Descent with modification and divergence into several lines 
is therefore certainly possible within the species. 
Comparative anatomy reveals the existence of similar plans of 
structure in large groups of organisms. Plants may have vegetative 
leaves, and in some cases these are modified into parts of flowers. 
Vertebrate animals have forelimbs that may be used for walking, run- 
ning, swimming, or flying, but in which the various parts of the skele- 
ton correspond, bone for bone, from the upper arm to the last joints 
of the fingers, whether the animal is a frog, a lizard, a turtle, a bird, 
a rabbit, a seal, a bat, or a man. This is what is meant by saying 
that such structures are homologous, and these correspondences are 
inexplicable unless the animals are descended from a common ancestor. 
Fundamental resemblance is therefore evidence of genetic affinity. 
The study of comparative behavior proves that related forms show 
gradations in their instincts, such as shamming death in insects and 
nest building in birds. At the same time, related species inhabiting 
different parts of the earth under very different conditions retain simi- 
lar instincts. Examples are the habit of thrushes in England and in 
South America of lining nests with mud, and that of wrens in England 
and North America of the males building “cock nests.” Why should 
this be, unless the different species of thrushes and wrens are descended 
from common ancestors in each case? 
Embryology reveals remarkable similarity in structure between 
young embryos of animals which in the adult stage are as different as 
fish, lizard, fowl, and man. This similarity even extends to such de- 
tails as the manner in which the blood vessels run from the heart to 
the dorsal aorta, a plan which is of obvious significance in the case 
of the fish that breathes by means of gills, but not so obvious in that 
of lizard, chick, or man, where gill pouches are formed in the embryo 
but soon become transformed into different structures, and breathing 
is carried out by other means. This similarity between embryos is ex- 
plained by the affinity and descent from a common ancestor of the 
groups to which they belong. 
Embryology also provides evidence of vestiges of structures which 
once performed important functions in the ancestors but now either 
perform different functions or none at all. Examples of such organs 
are the teeth of whalebone whales, the limbs of snakes, the wings of 
ostriches and penguins, and the flowers of the feather-hyacinth. Since 
Darwin’s time countless other examples have been discovered. The 
most striking of these are the pineal gland which is a vestigial eye, and 
vestiges of the egg tooth still found in marsupials, although it is 75 
million years since their ancestors had to use an egg tooth to crack 
the shell and hatch out of their eggs. Here again, descent from com- 
mon ancestral forms explains all these cases. 
