632 



ORGANIC EVOLUTION 



niches in the environment, keeping well out 

 of the way of the numerous reptiles. All the 

 while their bodies were evolving to a stage 

 where, when the opportunity for survival 

 improved, they were ready to set forth on 

 the road that led them to the dominant 

 position among present-day animals. Cer- 

 tainly the mammals have become the most 

 diversified of all animals, whereas the birds 

 have clung pretty much to a common pat- 

 tern that seems to serve them well for life 

 involving flight. Mammals reached their 

 peaks in the Tertiary and have since shown 

 a steady decline in numbers of species even 

 though man, a member of this group, is 

 considered to be the dominant form of 

 life on earth today. 



All of the evidence from studies of an- 

 cient life confirms and elaborates the theory 

 that living things today descended from 

 similar but different forms hving in past 

 ages. In other words, paleontological studies 

 confirm and extend the theory of evolution 

 as accepted by all scientists and most others 

 today. Let us continue our search for facts 

 supporting this or any other theory by ex- 

 amining evidence that appears in our living 

 world of today. 



EVIDENCE FROM RECORDS 

 OF LIVING ANIMALS 



We have seen that paleontology lends 

 substantial support to the theory of evolu- 

 tion, but what about the living world of 

 today? Can we find additional evidence 

 among the animals around us to add to the 

 bulwark already substantiated? Zoologists 

 have been busy for the past 150 years add- 

 ing vast stores of information to this end, 

 and today we find supporting evidence 

 from many branches of the biological sci- 

 ences. Indeed, so much has been accumu- 

 lated that it is questionable whether evolu- 

 tion should be referred to any longer as a 

 theory. It is rather a clearly established 

 fact. Some of the evidence which has af- 

 forded proof we shall consider briefly. 



From embryology 



The German biologist, Ernest Hendrich 

 Haeckel, many years ago, formulated the 

 Biogenetic Law based on the fact that all 

 early vertebrate embryos show remarkable 

 similarity in their early stages of develop- 

 ment. He claimed that animals recapitulate 

 in their early embryological stages the phy- 

 logenetic history of the race, which we 

 have already seen in various animals. 

 Haeckel's all-inclusive statement has since 

 been attacked from many sources and 

 today stands only in its skeleton form. 

 Although there is definite resemblance be- 

 tween the embryos of related species (Fig. 

 25-9), they are not as clear-cut and abso- 

 lute as was once thought, and this is as one 

 might expect. Since evolution has come 

 about by a series of many small changes 

 ( micromutations ) in the genie constitution 

 of a species, it might be expected that gene 

 changes would occur aflFecting all stages of 

 the individual, tliat is, gene changes might 

 affect the embryo itself so that "short cuts" 

 could be taken in producing a certain organ 

 system, for example. Similarly, structures 

 which no longer serve a purpose in the 

 adult animal might be retained simply be- 

 cause they were not "in the way" during the 

 course of development and because no spe- 

 cific gene change occurred that ruled them 

 out. Therefore, one might expect numerous 

 remnants lingering on in the bodies of some 

 animals but not in others, even closely re- 

 lated species. This is exactly what we 

 observe. 



In spite of some important omissions, the 

 parallelism between the developing indi- 

 vidual (ontogeny) and the phylogenetic 

 sequence of animals (phylogeny) is re- 

 markable and certainly lends support to 

 the theory of evolution. Both the fertilized 

 egg and single-celled Protozoa exist as indi- 

 vidual cells, the former destined to become 

 something more complex, whereas the lat- 

 ter never rises above the one-celled condi- 

 tion (Fig. 25-10). Colonial Protozoa (for 



