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Palaeontology being of little assistance in solving the history 

 of the local fauna, we have to fall back on temporary explana- 

 tions indicative of the various parts of the world whence 

 our local animals have come. For all species except resi- 

 dents this is comparatively easy, since we can readily trace 

 them back to their homes. Various insects and shells are 

 introduced by man from the different countries of the Con- 

 tinent, and the singular point about some of these intro- 

 duced species, is the fact of their very rarely, if ever, becoming 

 residents in the district. Thus, the Timberman Beetle 

 (Astynomits cedilis) occurs commonly at Middlesbrough in 

 imported timber and has even been taken in the neighbour- 

 hood Yet despite the very favourable plantations of fir 

 and pine this insect has not become established. 



Colonies of the Kentish Snail (Helix cantiana) introduced 

 in railway- ballast flourish for some time and then die out. 



Turning next to the migratory and visiting birds of Cleve- 

 land, these animals coming to us from distant countries 

 only need a change in their life conditions here, to make 

 them permanent residents. Doubtless in the past, changes 

 have taken place and converted a visiting species into a 

 resident species. 



It is generally accepted by naturalists, that all species of 

 animals have arisen by descent with modification from proxi- 

 mate or remoter ancestors. The species of a genus, however, 

 are not uniformly distributed. In some localities they are 

 .more numerous than in others, and if the species have 

 been derived from some common ancestor, it is clear that 

 where they are most thickly clustered the evolution of 

 forms will there have been most active. Gradually the 

 various individuals advancing further and further from a 

 centre of distribution give rise to less numerous forms. 

 and by tracing back our native species to their original 

 centres we can readily comprehend how they have dispersed 

 from different countries. 



The gradual approach of the Ice Age brought with it a 

 great number of Arctic animals and plants, some still surviving 

 in parts of our district. We may take as an example the 



