CHAPTER XXIV 



EVOLUTION 



General Discussions 



Nature of Evolution. — Evolution refers to the processes or 

 changes whereby new forms arise from previously existing forms. 

 According to the idea of evolution the organisms which first 

 inhabited the earth were extremely simple and from these the 

 more complex organisms have come. The first organisms to 

 inhabit the earth are supposed to have been single celled. They 

 were like the one celled Algae or the one celled animals we now 

 have. From these one celled organisms, others arose consisting 

 of more than one cell, and these were followed by organisms still 

 more multicellular. As organisms became more multicellular, 

 cells differentiated in structure and function and thereby mul- 

 ticellular organisms with tissues and organs appeared. These 

 were followed by organisms still more highly organized, and so 

 on the process continued, giving rise to all the various kinds of 

 plants and animals we now have. Throughout the study of 

 Thallophytes, Bryophytes, Pteridophytes, and Spermatophytes, 

 we traced the steps by which complex plants were evolved from 

 simple ones. There we noted the introduction of gametes; 

 differentiation of gametes into eggs and sperms; introduction of 

 sex organs; introduction of sporophyte generation; differentiation 

 of sporophyte generation into roots, stems, and leaves; introduc- 

 tion of sporophylls and strobili; and finally the introduction of 

 seeds and flowers. Within each group of plants there are various 

 degrees of complexity. For example, some Angiospersms are 

 much more advanced than others a,nd, as noted in their study, 

 they are grouped into orders and families according to an evo- 

 lutionary sequence. Evolution is usually progressive, giving 

 rise to better organized forms, but this is not always the case. 

 Sometimes evolution is toward simpler forms. For example, 

 the Fungi are simpler than the Algae from which they are sup- 

 posed to have come. They have no chlorophyll, have lost their 



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