THEORY OF EVOLUTION 487 



way of a toothed, feathered, extinct form known as Archaeopteryx. 

 It was essentially a flying reptile. The mammals probably descended 

 from the reptilian group Tlieromorpha by way of our modern mono- 

 tremes which lay eggs, hatch them out, and then suckle the young 

 with milk from mammary glands. The marsupials, such as kan- 

 garoos and opossums, are next in order, and from these it is thought 

 the Placentalia have arisen. Within this group some authorities 

 hold the view that the Primates, the order including man, have 

 arisen from Insectivora. The apes and monkeys belong to the Pri- 

 mate group, and there has been some misunderstanding among lay- 

 men generally in regard to the possible relationship of man and the 

 apes. Most people have the misconception that this is a linear 

 descent in which the most advanced member of the lower group 

 represents the immediate ancestor of the next higher group. As a 

 matter of fact, the theory is not that the higher monkeys are in 

 the process of becoming apes and the higher apes becoming men, but 

 that all three of these groups have had origin as different lines from 

 a common primitive form. 



Recapitulation Theory. — In the early part of the nineteenth cen- 

 tury von Baer observed that the early stages of vertebrate embryos 

 of different classes had a very close resemblance to each other. He 

 did not subscribe, however, to the recapitulation theory when it 

 was formulated later. Haeckel, coming a little later, became con- 

 vinced that the developing embryo lives over again the stages 

 through which its whole race has passed, and he formulated the 

 recapitulation theory or biogenetic law from this idea. In other 

 words, the organism in its individual life tends to recapitulate the 

 different stages through which its ancestors have passed in their 

 racial history. Briefly this same statement is, ontogeny recapitulates 

 phylogeny. The rehearsal of the phylogeny is in rather slurred form 

 in some details, but the basis for the idea is readily seen. In brief, 

 the theory is applied by comparison. Nearly all metazoan organ- 

 isms begin life by the union of the two germ cells to form a single- 

 celled zygote which is the new organism. At this time it is com- 

 parable to the Protozoa. During the ensuing cleavage divisions a 

 colonial form is represented. Following this, when one side infolds 

 to form a gastrula with two germ layers, the embryo is almost 

 identical to the diploblastic coelenterates as represented by Hydra. 

 Following this, the third germ layer forms between the others and 

 results in the triploblastic metazoan. 



