Introduction [ 3 



synthesis was established in certain organisms, aerobic energesis became possible both 

 to these and to others. This made possible a manner of life more vigorously active 

 than before. The inconsiderable groups of autotrophic bacteria — the organisms which 

 live by oxidizing inorganic matter — appear to be secondary developments dependent 

 upon the existence of photosynthesis. 



The organisms whose origin has been suggested thus far — the ordinary bacteria, 

 anaerobic and aerobic, the autotrophic bacteria, the purple bacteria, and the blue- 

 green algae — are relatively simple in structure and function; all consist of minute 

 physiologically independent cells. The first step in the evolution of more complex 

 organisms was the evolution of the nucleus. 



Morphologically, the nucleus is a part of a protoplast which is set apart by a mem- 

 brane and which originates ordinarily by division of a pre-existent nucleus in the 

 manner called mitosis. In this process, a definite number of definite chromosomes 

 appear and undergo equal division. The nucleus exercises control over the protoplast 

 in which it lies. Its controlling action depends upon the chromosomes which go into 

 it, and mitosis has the effect that all nuclei which are derived from one original nu- 

 cleus strictly by normal processes of mitosis are identical in the controlling effects 

 which they exert. Thus the nucleus serves for the precise transmission of a compli- 

 cated heredity. Beside mitosis, there are two other processes — two only — meiosis and 

 karyogamy, by which nuclei may produce other normal and enduringly viable nuclei. 

 In a sequence of generations of individuals sexually produced, these processes occur 

 alternately, each one at one point in each cycle of sexual i-eproductlon. Mendelian 

 heredity is produced by changes, in the sets of chromosomes (or parts of chromo- 

 somes) in individual nuclei, which occur during meiosis and karyogamy. The role of 

 the nucleus in sexual reproduction is one of its essential characters: the nucleus is re- 

 lated to sexual reproduction, including Mendelian heredity, as structure to function. 



The existence of organisms without nuclei shows that the nucleus evolved after life 

 did: it did not evolve at the same time as protoplasm. The essential uniformity of 

 the nucleus and of its association with sexual reproduction shows that these things 

 evolved only once, and together. There are a very few organisms, as Porphyridium 

 and Prasiola, in which the presence or absence of nuclei is not certain; there is ac- 

 cordingly scant evidence for speculation as to the manner of this evolution. As to the 

 tim.e, we know only that microfossils representing nucleate organisms occur in the 

 uppermost strata of the Proterozoic era. 



By making possible the precise transmission of a complicated heredity, the nucleus 

 has made possible the development of complexities of structure and function exceed- 

 ing by far anything occurring in non-nucleate organisms. It appears that as soon as 

 the nucleus was in existence, organisms provided with it entered upon evolution in 

 many characters and gave rise to many distinguishable groups. Among these groups, 

 those which consist respectively of the typical plants and the typical animals are the 

 greatest. There is, however, neither any a priori reason, nor any evidence from nature, 

 for a belief that all groups of nucleate organisms must naturally belong to one or the 

 other of these two. Several other groups, in general much less considerable than these, 

 are thoroughly distinct and appear equally ancient. 



E. B. Copeland understood the history of life very much as it has just been pre- 

 sented. In his teaching, he treated the bacteria and blue-green algae as standing en- 

 tirely apart both from plants and from animals, and pointed out several other groups 

 which are not as a matter of nature either plants or animals. It was his opinion that 

 these groups should be treated as a series of minor kingdoms; he excused himself 



