14 INTRODUCTION [ CH - l 



plasy, whereas resemblances believed to indicate blood-relationships 

 are grouped under the term homology. 



Again, the immature forms of some animals are found to exhibit 

 strong resemblances to the adults of others, and the eggs of all the 

 highest animals show the strongest general resemblance to the 

 simplest animals the so-called Protozoa (Gr. TT/XOTOS, first, o>oi/, 

 animal). If these resemblances are to be interpreted in the same 

 way as those prevailing between adults and it is illogical to refuse 

 to do so then we are driven to conclude thart most animals in their 

 development pass through stages when they exhibit many characters 

 once possessed by their ancestors, commencing at 'the stage of the 

 Protozoa. Some of these latter animals, since they are about as 

 simply constructed as we can imagine living matter to be, may be 

 looked on as slightly modified survivors of the first animals which 

 appeared on the globe. 



This method of interpreting the changes which occur during 

 development is what is known as the Recapitulation Theory, 

 because during Ontogeny (Gr. ov, OVTOS, being) or the development of 

 the individual, nature recapitulates to some extent the development 

 of the species in past time, Phylogeny (<vXov, a stock, a race). 

 There are, however, a great many other factors which have modified 

 development, and the determination of these and their separation 

 from the hereditary factor is a task requiring careful study and one 

 which is as yet far from complete. 



