472 AN INTRODUCTION TO ZOOLOGY 



crossing over occurs are investigated, it is found that there is a very 

 definite relation between the percentage of crossing over in groups 

 containing a certain number of common factors. Let us consider 

 an imaginary example. If a is a factor that is usually linked with a 

 group, but sometimes crosses over, then we might find in a cross 

 that a would remain linked with the group in 95 per cent, of the cases, 

 while it would cross over in 5 per cent. Thus we could term 5 the 

 cross-over value and 95 the linkage value. Suppose, further, that 

 in crossing over a is sometimes accompanied by other factors, 

 b and c, then of course there would be a cross-over value for each 

 combination. Suppose the cross-over value of the pair ab is 5 

 and of be is 10, both pairs containing a common factor b, then we 

 shall find that the cross-over value of a-c is either a function of the 

 difference between the values of ab and be, i.e. 5, or a function of 

 their sum, i.e. 15. This is so unless the results are interfered with 

 by some other phenomenon, and it is a condition that, as may be 

 seen, would follow if the factors were regarded as three points on a 

 straight line. If we take four factors or five factors, and so on, this 

 inter-relationship still holds good, and, again, this is readily compre- 

 hensible if they are arranged in a straight line. Thus we can deduce 

 a further generalisation of the inheritance of factors, namely, that 

 such factors, responsible for the production of certain characters, 

 are arranged in a linear order. This is sometimes referred to as the 

 law of the " Linear order of the factors.' ' It should be noted that this 

 result follows from an analysis of the mathematical results obtained 

 by experiments, and is independent of any consideration of chromo- 

 somes, etc., but nevertheless it is obvious that the chromosomes 

 and their behaviour in maturation and fertilisation provide a good 

 mechanism by means of which we can conceive that such a linear 

 arrangement of the factors with its attendant phenomena could 

 easily be carried out. 



We may here leave the question of hereditary relationship 

 between parent and offspring and return to the more general 

 question of the manner of evolution of the various forms of life by 

 considering the outlines of the past history of one group of animals 

 as far as it has been revealed by Palaeontology. For an example we 

 shall take the Class REPTILIA, since they are fairly well known, and 

 from them have sprung the higher vertebrates living in the world 

 to-day. 



Somewhere near the middle of the Devonian period the primitive 

 pentadactyl Vertebrates had already appeared on the Earth. Various 

 authorities have calculated how long ago this was, and arrived at very 

 different estimates, but even low ones give it as over 20,000,000 



