The number and kinds of organism is not constant. How do you explain this statement ? 

Explanation

For any organism population can not remain constant. In some of generations, population can be more because of favorable condition. In some other generations, population can be less because of unfavorable conditions. The kinds of organism cannot be constant because evolution is a never ending process. In every era new species come into origin and some old species become extinct. For example, number of mosquitoes are very large in warm and humid weather, whereas in extreme summer or winter they can be seen rarely.