GUIDING FACTORS. INTRODUCTION 345 



and that challenge explanation. The following chapter on "Adapta- 

 tion in Nature" is frankly an interpolation, but an essential one, unless 

 the student be already well informed in this field. Also the chapter on 

 "The Web of Life" is necessary if we are to avoid thinking of species as 

 isolated entities. The evolution of a species always takes place in a 

 natural setting, part of which consists of the lifeless environment and 

 part of which consists of its animal and plant neighbors. 



Orthogenetic trends. — Various kinds of definitely directed evolu- 

 tionary trends have been described. Sometimes these trends concern 

 themselves with such relatively trivial characters as the intricate mark- 

 ings on the shells of mollusks and brachiopods, sometimes they have 

 to do with the horns of elks or of titanotheres, and sometimes they 

 concern the toes and teeth of the horse family. One striking feature 

 of these so-called "orthogenetic" series is that the trends have not al- 

 ways culminated in a condition of maximum efficiency, but have some- 

 times gone beyond this point and have become positively detrimental, 

 in some cases even contributing to the extinction of the species possess- 

 ing them. If there is some guiding factor other than natural selection 

 that tends to push evolutionary changes forward in definite paths, such 

 a force might be responsible for adaptive trends as well as the non- 

 adaptive ones. In general, however, it may be said in advance of the 

 more detailed discussion of this question in a later chapter, that most 

 geneticists are either skeptical of the existence of any very definite 

 orthogenesis, or else consider it explainable through natural selection. 



Progressive evolution. — It is generally assumed that "evolution" 

 and "progress" are synonymous terms. It is claimed that forms of 

 life now living are on the whole "more advanced" than those that have 

 lived in earlier times and that there has been a steady advance through- 

 out the ages. But what do we mean by progress in evolution or by 

 more or less advanced forms? Progress and advance imply both direc- 

 tion and a goal or goals. What are the goals toward which evolution 

 is progressing? 



Some claim that the final goal of life is perfect adaptation of all 

 living things to the environment and to one another. But is there any 

 evidence that life today is any better adapted than was life in Cambrian 

 times? Is there any ground for assuming that man is any better adapted 

 to the environment than is the Amoeba? Man is adapted to a much 

 more complex environment than is the Amoeba, but is he any better 

 adapted to the human environment than is Amoeba to the amoeban 

 environment? I see no reason for assuming that there is any difference 



