ANIMAL EVOLUTION 



5 z 9 



modification which is due to conditions of 

 environment. 



This definition is much too narrow. It 

 is simply a slight broadening of a defini- 

 tion of an individual, and contemplates 

 primarily material in museum collections. 

 Furthermore, it is based primarily upon 

 vertebrates, especially birds and mammals, 

 and scarcely applies to the other forms of 

 life even as represented in museums. 



What, then, is a species? That is 

 difficult to say, for the different kinds of 

 animals vary greatly in the interrelation- 

 ships of the individuals and in their 

 relations to allied types. 



A species must be considered as repre- 

 sented by the so-called normal type plus 

 all of its variants and aberrations, whether 

 occurring naturally or induced by changed 

 conditions. 



In any kind of animal the normal type, 

 so called, at every stage represents merely 

 the present actuality. The variants and 

 aberrations represent the potentialities, 

 and with changed conditions any one or 

 more of these may become an actuality, 

 replacing the normal form, representing 

 it in another region, or representing it in 

 the same region under different ecological 

 conditions. 



THE THREE PATHS OF DEVELOPMENT 



No true appreciation of animal life 

 taken as a whole is possible without a 

 consideration of animal development. 

 All animals originate from single cells. 

 Some remain all their lives as single cells, 

 while in others the original single cell 

 becomes a more or less complex mass of 

 cells. 



Assuming that the earliest animals, 

 like those of the present day, began life as 

 a single cell, there are three alternatives 

 which subsequent development might 

 follow. There is no reason for believing 

 that these three paths were not followed 



simultaneously, that is, that animal life 

 did not from the first develop in three 

 divergent ways. 



A single cell cannot increase in size 

 beyond a certain point without serious 

 interference with the chemical and physi- 

 cal interchanges on which life depends. 

 On reaching the maximum size permitted 

 by the chemical and physical restrictions, 

 the animal cell divides into two; later 

 these two divide each into two, becom- 

 ing four, these four become eight, these 

 eight sixteen, these sixteen thirty-two, and 

 so on indefinitely. 



In this process of division there are three 

 paths that may be followed. As they 

 divide the cells may separate from each 

 other so that the individual animals 

 always remain composed of a single cell. 

 In other words, on the division into two 

 of the original cell each half may separate 

 from the other and become a separate 

 animal half the size of the original. 

 Further division would give rise to a 

 corresponding number of entirely separate 

 animals, all when they reach the maxi- 

 mum size increasing by simple division 

 into two. The so-called single celled 

 animals, or protozoans, illustrate this 

 process. 



But after division into two the cells 

 might remain in contact, and this contact 

 might be maintained through successive 

 cell divisions. Here there are two alter- 

 natives. The cells may adhere more or 

 less irregularly so that a poorly differen- 

 tiated mass of cells results, the mass as 

 a whole being more or less distinctly 

 radial in symmetry. The result of such 

 development is represented by the sponges. 



But on the other hand cell division may 

 take place by regular geometrical progres 

 sion, the original cell dividing into two, 

 four, eight, sixteen, thirty-two, and so on, 

 until a hollow ball of cells (a blastula) is 

 formed, which, by collapsing, would form 



