THE CHANGING WORLD OF LIFE 559 



but the center toe is enlarged and the other two did not touch the ground. 

 The most recent fossils show a horse about the size of those today and 

 using only one toe, which was greatly enlarged. Similar progressive 

 changes can also be observed in the length of the skull and the de- 

 velopment of the teeth. Fig. 34.14 shows some of the changes that have 

 been noted. 



The development of many other animals has been worked out in a 

 similar manner, but the horse offers the most complete series. The 

 evolution of the elephant was worked out with the aid of complete pre- 

 served specimens in addition to fossils. A number of years ago, ex- 

 plorers in the frozen wastes of northern Siberia noticed their dogs eating 

 something just under the snow. Investigation showed that it was the 

 frozen body of a huge mammoth that lived about 20,000 years ago. 

 This was a huge, hairy ancestor of our modern elephants that lived in 

 temperate and cold climates. Since that time about fifty of these pre- 

 historic animals, preserved by cold storage, have been found. As stated 

 previously, it is even reported that there was a scientific banquet in 

 London at which 20,000 year old mammoth steaks were served. 



Evidence from Serology. Serology is one of the most recently de- 

 veloped studies which gives us evidence of evolution. The technique 

 employed, in brief, is as follows. The liquid part of the blood, the 

 serum, of a certain animal is separated from the blood cells and injected 

 into some other animal, such as a rabbit. This causes the test animal to 

 develop antibodies against certain protein components of this serum. 

 This is the same sort of reaction which causes animals to develop anti- 

 bodies against disease germs. The human body also sometimes de- 

 velops antibodies against certain pollens which may be inhaled, and the 

 condition known as hay fever results when this pollen reacts with the 

 antibodies. Now, let us see how this sort of reaction helps in studies 

 of evolution. Suppose the serum from domestic cattle is injected into 

 a rabbit. If some of the serum from the rabbit is now mixed in a test 

 tube with some of the serum from a domestic cow, there will be a precipi- 

 tation and the mixture will become cloudy. This gives visual evidence 

 of the reaction of the antibodies. Now, suppose we mix some of the 

 serum from such a rabbit with the serum from the bison, or American 

 buffalo. We would probably get some cloudiness, but not as much as 

 in the first instance. This would indicate that there was some degree of 

 relationship between these two species. If we now mixed some serum 

 from this same rabbit with serum from an elk we might get a slight 

 precipitation, but not as much as from the bison. This would indicate 

 a more distant relationship. If mixed with the serum from a horse, 

 there would probably be little or no precipitation. 



