404 GENERAL BIOLOGY 



the change to be transmitted. For example, a giraffe by constantly eat- 

 ing food from trees, finds it necessary to reach higher and higher. This 

 stretching of the neck will, then, in each generation cause the young 

 giraffes to be born with a slightly longer neck. 



If such a change makes the individual better able to adapt itself to 

 its surroundings and thus gives it an advantage in the struggle for exist- 

 ence, it is said to be a selective factor. 



The mutation theory, contrary to the Darwinian, insists on sudden 

 jumps or great changes taking place which are then transmitted. This 

 theory is based on the fact that there are so-called "freaks" or "sports" 

 in nature which suddenly spring forth. 



The crooked-legged sheep is the classic example. A New England 

 ewe gave birth to a peculiar crooked-legged ram. The shrewd Yankee 

 farmer, who owned the sheep, saw in this crooked-legged ram an animal 

 that could not jump fences, and so kept it. The crooked-leggedness 

 proved to be a Mendelian dominant character which is transmitted from 

 parent to offspring. There are now great numbers of the descendants 

 of this single New England crooked-legged ram, which are in turn 

 crooked-legged. 



Such mutations can be explained by assuming that the recessive 

 characters in egg and sperm have met after lying dormant for many 

 centuries. 



In fact, there are a number of examples of characters lying dormant 

 and being carried on from parent to offspring, only coming forth at cer- 

 tain times when the mating organism likewise has a similar dormant 

 or recessive character. For instance, many mullein plants will, in certain 

 years, suddenly produce longer leaves than is normal for that plant, thus 

 showing that the cause of the longer leaves must be in the germ plasm 

 of the varying plants. For if such were not the case, there would not 

 be so many to develop longer leaves in the same generation. 



In other words, this means that there is a peculiar arrangement of 

 genes in the chromosomes of the mating-plants, and these genes have 

 united to cause a similar abnormal development in all those plants which 

 spring from similar zygotes. 



If is for reasons of this kind that biologists have come to the con- 

 clusion that environment does not change the organism to any appre- 

 ciable extent in its genetic value, but, that whenever changes come forth, 

 these are due to changes in the germ plasm. 



To the two great theories mentioned above, there have been added 

 at various times what might be called sub-theories or part-theories to 

 account for certain developmental characteristics. The most prominent 

 of these is known as Orthogenesis. Eimer and Nageli are its sponsors. 

 Orthogenesis means that there is something in the organism which, once 

 a line of development has begun, this something will enable the organism 

 to continue in that certain line of development even though it kill the 



