Evolution

Introduction

The results of artificial selection have resulted in extremes in size varying from the Great Dane to the Chihuahua; and extremes in form such as the short-legged dachshunds, and the English bulldog with a short snout and compressed face. Some of these extreme traits have lead to health problems in some breeds.

Early Theories

Hippocrates thought that all aspects of nature could be traced to their underlying causes. The early naturalists, such as Aristotle, believed that nature was a continuum or organization from lifeless matter through complex forms of plant and animal life. He thought that each organism was distinct from all the rest and nature was a continuum of organization. By the fourteenth century this had been formalized into the idea of the great Chain of Being in which living organisms were unchanging links.

There was, however, confounding evidence from biogeography, comparative anatomy, and geologic discoveries.

Scholars began to examine the world distribution of plants and animals, a study called biogeography, and learned of the tremendous variation on the earth. Biogeography showed that exotic species from distant lands did not seem to "fit" into the Chain. The size of the known world expanded enormously in the 15th century. The discovery of new organisms in previously unknown places could not be explained by accepted beliefs. People questioned how species got from the center of creation to all these places?

Comparative morphology is the study of similarities and differences in body plans of major groups. It revealed some puzzling patterns. Studies of comparative morphology revealed similarities in th e bones of animals not considered to be related. Animals as different as whales and bats have similar bones in forelimbs. Some animals possessed body parts for which a function no longer existed (for example, pelvic girdle bones in snakes [python skeleton on left, human skeleton on right), but these anomalies revealed a relationship to other animals.

By the mid-1700s, geologists were beginning to map the layering of the earth's crust. They found similar rock layers throughout the world. Certain layers contained fossils. Deeper layers contained simpler fossils than shallow layers. Some fossils seemed to be related to known species.

Fossils revealed that changes had occurred in organisms over periods of time or had evolved. It became clear that the distribution of fossils around the earth was an argument against a single time and place of creation.

REVIEW: Fossils found in the lowest geological strata are generally the most
Answer primitive.