XII, XIII 



THE ARRAY OF LIVING ORGANISMS 



(Readings: Weisz, Chapter 29. S.P.T., Chapter 19. Villee, pp. 83-84. R. Y. 

 Stanier, M. Doudoroff, and E. A. Adelberg, The Microbial World, Prentice- 

 Hall, 1957, Chapters 3-6. Other readings are listed at appropriate places within 

 the exercise.) 



So far this semester we have studied general 

 properties and processes in living organisms, 

 emphasizing components and reactions rather 

 than the specific organisms in which they were 

 examined; and so we shall go on doing. Yet 

 we have already encountered a fair variety of 

 organisms, and next semester we shall deal with 

 many more. Also next term we shall study more 

 highly integrated phenomena as they appear in 

 more complex organisms. 



Up to now, whenever we have encountered a 

 new organism, something has been said of its 

 biological position. The time has come to go 

 beyond such a piecemeal approach and to gain 

 a view of the entire array of living things. 



Biological order as history and experiment 



At first glance nature appears to abound with 

 an enormous diversity of living organisms. 

 Careful examination of the different forms, how- 

 ever, enables us to group them on the basis of 

 similarities in anatomical organization, embryo- 

 logical development, chemical constitution, and 

 other criteria. The catalogue of types that 

 results is highly useful in itself, both in reducing 

 the diversity to manageable proportions and in 

 enabling us to ascertain readily the general na- 



ture of an organism once we are aware of its 

 name or of enough of its properties to place it 

 in the classification scheme. 



But emerging from this classification, and 

 indeed woven inseparably into its fabric, we 

 find two tremendous concepts: that of origin 

 and descent; and that of progressive adaptation, 

 of ceaseless problem-solving, accompanying the 

 ceaseless expansion of organisms into every 

 environment that can support life, working out 

 in all environments their universal problems of 

 nutrition and reproduction. That is, we come 

 out not only with a history, but one that in- 

 volves direction, indeed many simultaneous di- 

 rections; in essence, the history of the explora- 

 tion of this planet as an abode for life. It is 

 this that transforms what might otherwise be a 

 tedious catalogue into a profound intellectual 

 adventure. 



The dominant view that guides the construc- 

 tion of a scheme of classification — once one has 

 simply made order, grouping similar organisms 

 together and separating the groups in proportion 

 to their differences — is that all living organisms, 

 plant and animal alike, are linked together by 

 descent from common ancestors, from which 

 they evolved along separate paths to their pres- 

 ent state of divergence. Although each group 



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