152 CONCLUSIONS 



could have given rise to both the Protozoa and the Metazoa. 

 Here again nothing definite is known. We can believe that 

 any one of these views is better than any other according to the 

 relative importance that we accord to the various pieces of 

 evidence. 



(5) The fifth assumption was that the various invertebrate 

 phyla are interrelated. If biogenesis occurred many times in the 

 past and the Metazoa developed on several finite occasions then 

 we might expect to find various isolated groups of invertebrates. 

 If on the other hand biogenesis was a unique occurrence it should 

 not be too difficult to show some relationship between all the 

 various invertebrate phyla. 



It should be remembered, for example, that though there are 

 similarities between the cleavage patterns of the eggs of various 

 invertebrates these might only reflect the action of physical laws 

 acting on a restrained fluid system such as we see in the growth of 

 soap bubbles and not necessarily indicate any fundamental 

 phylogenetic relationship . 



As has already been described, it is difficult to tell which are the 

 most primitive from amongst the Porifera, Mesozoa, Coelenterata, 

 Ctenophora or Platyhelminthia and it is not possible to decide 

 the precise interrelationship of these groups. The higher 

 invertebrates are equally difficult to relate. Though the concept 

 of the Protostomia and the Deuterostomia is a useful one, the 

 basic evidence that separates these two groups is not as clear cut 

 as might be desired. Furthermore there are various groups such 

 as the Brachiopoda, Chaetognatha, Ectoprocta and Phoronidea 

 that have properties that lie between the Protostomia and the 

 Deuterostomia. It is worth paying serious attention to the con- 

 cept that the invertebrates are polyphyletic, there being more than 

 one line coming up to the primitive metazoan condition. It is 

 extremely likely that the Porifera are on one such side line and it is 

 conceivable that there could have been others which have since 

 died away leaving their progeny isolated; in this way one could 

 explain the position of the nematodes. The number of ways of 

 achieving a specific form or habit is limited and resemblances 

 may be due to the course of convergence over the period of many 

 millions of years. The evidence, then, for the affinities of the 

 majority of the invertebrates is tenuous and circumstantial; not 



