BASIC ASSUMPTIONS 17 



that solar systems similar to our own have been created many 

 millions of times and thus conditions favourable to life may be 

 present on many other planets in the universe. Shapley (1957) 

 has calculated that there are probably 10 8 planets that have con- 

 ditions favourable for life of one sort or another. We have no 

 evidence that living things can be transmitted between the stars 

 and still remain alive on reaching their destination, but it would 

 seem that we shall shortly have information on transmission of 

 living material by rockets. It would perhaps be more to the point 

 to have information concerning transmission by meteorites. 

 Oparin (1957) gives quite an extended discussion of meteorite 

 transmission of life and concludes that it was, and still is, highly 

 improbable. Nevertheless, at present we have very little informa- 

 tion on this subject and it is likely that the renewed interest in 

 space travel will stimulate further investigation into the nature and 

 properties of meteorites. 



Unique occurrence of life. The second suggestion is that 

 though we are unable to show at present that life is formed de 

 novo on this earth, there is no evidence to show that when life was 

 formed on this earth it was a unique event. Haldane (1954) and 

 Oparin (1957) are of the opinion that life was uniquely formed, but, 

 as they both point out, nothing is definitely known about what did 

 happen; all is hypothesis, and though it is simpler to assume 

 that it was a unique occurrence there is no reason why this simple 

 explanation should be the correct one. In the previous pages it 

 has been pointed out that our knowledge of the cell metabolism 

 is insufficient to allow us to state categorically that all cells in all 

 living forms have the same biochemical systems at work. Though 

 the similarities are often great, the dissimilarities may be just as 

 impressive. 



If living material had developed on several different occasions 

 or at different places at the same time, then one would expect to 

 have a large number of distinct groups of animals, whose relation- 

 ships and affinities are difficult to determine. 



This, as we shall see, is the present situation. 



