We are happy for the question you have.
According to our scriptures there are 8.4 million varieties of species. It means that all the moving creatures have been classified into 8.4 millions. In each of them there again may be sub-divisions viz., if you take a cat for instance, there are white cats, black cats and cats in different verities. That is how we take them.
Now, a soul after leaving a body need not enter the same variety of body. It may enter any body. But, that depends upon the karma it is carrying with it. That karma leads one soul from the bottom to the top and from top to the bottom also. That means, a soul may go to an insect body from a human and a tiny creature may come and take a human body also. Kingdom of each creature has its own qualities, but that is decided only after getting that particular body by the soul. All this is about moving creatures only.
The soul may enter even plant body also depending upon its own karma. Because we are able to prove the existence of life in all types of plants and trees also. Your friend need to understand a lot in this subject.