

United Status iJuieuu of Aniiudl Industry 



OBSTACLES TO LIVING 



Two herds of cattle of the same age and the same breed were supplied with abun- 

 dant food. One, however, was infested with ticks, which interfered with the nutrition 

 and health and growth of the animals 



How Do Different Kinds of Cells Arise? 



Variations in Protoplasm^ The individual has grown from a single 

 cell. This has given rise to more and more protoplasm. It has divided into 

 more and more cells. There come to be many different parts, many different 

 tissues, each having distinct qualities (see illustration, p. 348). Hovv^ can the 

 growing body be constantly changing and still remain the same individual ? 



One way of answering the question is to say that there is really no im- 

 portant change between the egg and the later stages; there only seems to be. 

 That is, the chicken has always been in the egg, only too small for us to 

 recognize. If we open a swollen bud in the spring, we can see the tiny 

 leaves, which merely enlarge and unfold as they absorb water: nothing 

 changes. The oak tree is already present in the acorn, and growing up is 

 merely an expanding, an unfolding. In most seeds we can actually see the 

 distinct parts of an entire plant — root, shoot, and leaf. 



Preformation This idea that the organism exists — in miniature — in 

 the egg and merely unfolds as it grows is a very old one and appeals to many 

 people as quite reasonable. Through many centuries people thought that 

 pre-formation was the correct explanation — that everything that the indi- 



iSee No. 1, p. 364. 



346 



