ly human characteristics. This is not equivalent to saying that the expert embryologist is in any doubt as to the diagnosis of a human embryo no matter how early the stage, for there are specific features about all embryos from the egg stage on to the end of development that may be distinguished by any one sufficiently versed in the subject. In spite of these specific differences, however, there can be no question that the embryology of man and that of any of the anthropoid apes show the closest of resemblances at every stage and diverge sharply only in the late stages of prenatal life. So close a resemblance in developmental histories is found only in species that are members of the same ancestral stock, for they have both inherited the characteristic features of their development from their common ancestors.
The evidence of human evolution as derived from a study of embryology is in no wise exceptional; in the contrary, it is quite typical and may be taken as indicating that from the developmental standpoint man is at one with other animals.
Evidences from Paleontology
Paleontology is the science of ancient life. Its materials are the more or less complete preserved remains of animals and plants that once lived. We call those remains fossils. Fossils are real; they cannot be explained away. If evolution has taken place and samples of every species that has lived were preserved for study, it would still be a task of immense difficulty to work out the pedigrees of all types of organisms now living, and we might still be largely in the dark as to the causes of the observed changes. As it is, we have fossil remains of perhaps only about one out of each thousand extinct species, a mere random sampling of the types that prevailed during the various past ages. Considering how many factors have been at work to prevent fossilization of large groups of species and how erosion and metamorphosis have worked together to destroy those fossils already preserved, we marvel that our fossil record is sufficiently complete to tell any sort of sequential story. The fact is that the record is surprisingly full and rich.
Age of the Earth
According to the most recent computations based on the rate of radium emanation, 1,000,000,000 years have elapsed since the earth attained its present diameter. Various estimates as to the time since the first life appeared upon the surface of the globe range from 50,600,000 years to about ten times that figure. Even the lowest figure gives ample time for any sort of evolutionary change, no matter how slow.
The Earth's Strata as Time Markers
The crust of the earth is arranged in a series of horizontal strata of varying thickness. The lowest layers are obviously the oldest, except in a few localities where breaks and tilts have occurred. Even in the most disturbed mountainous regions it is an easy task for the geologist to determine the original order of the strata.
First—None of the animals of the past are identical with those of the present. The nearest relationship is between a few species of the past which have been placed in the same genera as those of today.
Second—The animals and plants of each geologic stratum are at least generically different from those of any other stratum.
Third—The animals and plants of the oldest geologic strata represent all of the existing phyla except the vertebrates, but the representatives of the various phyla are relatively generalized as compared with modern representatives of the same phyla.
Fourth—There is a general progression toward more highly specialized forms as one proceeds from lower to higher strata.
Fifth—Many groups of animals reached the climax of their specialization long ages ago and have become extinct.
Sixth—Only the less specialized relatives of those most highly specialized types survived to become the progenitors of the modern representatives of the group.
