MODULE 2

EVOLUTIONARY THEORY

QUESTIONS TO BE ANSWERED IN MODULE 2

LAMARCK, DARWIN, AND VARIATION

The Darwinian revolution suggested four major ideas: 1) gradual changes take place in a species; 2) man evolved from some lower form of animals; 3) those organisms that adapted to a particular environment will survive; 4) the environment selects for survival which species or which factors in a species (e.g. intelligence, will, exercise, strength) are favorable. The first generalization leads to the second; and the third leads to the fourth. The last two points represent the ma;or interpretations of survival. Change the environment to meet the needs of the individual or recognize that inheritance or special advantage permit some individuals to excel or survive in a particular environment.

Lamarck

But what accounts for the modification of or the variation within species or between related species? Lamarck had assigned a more active role to the behaving organism. In the process of adapting to the environment, the animal would necessarily exercise, resulting in enlarged muscles or change structures. These changed structures were acquired characteristics and were passed on to subsequent generations. Use or disuse of parts resulted in the elaboration or elimination of structures. Lamarck's first law (1809) stated that: "In every animal ... a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ ... while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears (Bartlett, 1968, p. 475)."

By this he meant that certain structural changes in one's ancestors could be passed along. If one generation of people exercised a finger and the muscles grew larger as a result, this anatomical change could be passed on to subsequent generations. Giraffes were believed to have long necks because they had to stretch to get the bananas out of the trees. And each generation stretched a little more and passed that difference down to subsequent generations. Conversely, if certain structures were not used (like tails of humans) then they might not be passed on in later generations. The acquired characteristics were really the products of activity which somehow changed the genetic code and was passed along. McDougall and other psychologists attempted to prove this theory by training rats in such a way that exercise produced a change of musculature; observations were then made to see whether this change cropped up in later generations. It did not. This procedure is not to be confused with selective breeding, where bright rats have been bred together to produce a strain of bright offsprings. Acquired characteristics assume that changes produced in one generation are passed on to subsequent generations inherited; selective breeding is the mating of individuals with the same characteristics and observing if it occurs in the offspring.**

Lamarck held that there were progressive changes in the development of a species. There were, more generally, progressive changes in the universe. This point of view, known as finalism, assumed that the universe was moving in the direction of something "better." What was really "better" is never quite clear, but most finalists have taken the position that the universe has been moving in the direction of humans or those things which humans value (Simpson, 1951, p. 108). Man develops by some unexplained impulse within him which produces gradual improvement. It is this impulse which accounts for systematic changes in the structure of the species. Such an optimistic belief in the eventual betterment of both mankind and the universe is found in the positions of the French Lamarckians, German nature-psychologie, and the Americans McDougall and Edward Cope. Erasmus Darwin, grandfather of Charles Darwin and of Francis Galton, also believed in purposeful struggle.

Inherent purposefulness in the universe rather than blind striving after survival is a central doctrine in most religions. Therefore, Darwin and modern evolutionists clashed with church leaders. Sometimes the discord between scientist and theologian was only semantic, as expressed in the following poem by W. H. Carruth, "Each in His Own Tongue:"

A fire-mist and a planet --
A crystal and a cell --
A jellyfish and a saurian,
And caves where the cave-men dwell;
Then a sense of law and beauty;
And a face turned from the clod --
Some call it Evolution,
And others call it God.

Darwin had observed wide differences among plant and animal species. These differences within any species cried for explication. There were two explanations for the variation -- causes from without or strivings from within. A second issue, whether variation tends toward a progressive development, was generally supported by the advocates of internal strivings. Erasmus Darwin, Lamarck, and McDougall all represented this group. They believed in progressive development -- man as but a part of an evolving universe.

The number of prominent vitalistic scientists gradually declined during the latter part of the nineteenth century. The last great vitalist, the physiologist Johannes Mueller and the evolutionist Herbert Spencer, could no longer hold out against their younger counterparts, Helmholtz and Darwin. Psychologists in the Helmholtz School of medicine turned to the models of physics and chemistry and relied only upon physical and chemical forces to explain biological phenomena. Transformation equations were being generated; energy changes from one kind to another was an empirical fact. Mechanical energy could be transformed into electrical energy and then into chemical energy. The interrelation among forms of energy is a major physiological explanation. The concept of force was challenged by the conservation of energy doctrine. Life force was unnecessary to explain animal functions.

Charles Darwin (1809-1882)Photo of Darwin

Darwin made contributions to this issue, through a program of careful observation, description, and analytical thinking. Darwin argued for continuous development of the species. He not only provided evidence for the gradual change in the species, but he went one step further and developed a theory to account for change. The theory went something like this: some characteristics of species survive because they are adaptive in a particular environment; others fade out because they are not adaptive. Why can't all variations survive? Because there is limited food supply, according to Malthusian theory. So you really have two possibilities: either 1) the amount of food equals or exce