Variations are grouped into three pairs of contrasting type.
1. Germinal or acquired, 2. Continuous or discontinuous, 3. Determinate or indeterminate.
1. Blastagenic or Germinal Variations
These are intrinsic which, apart from the external influences, arise in the germ plasma of the animals and plants. These may be caused by the recombination of genes which results into a group of new combination, or it may be caused due to mutation which is the result of some radical change in the gene complex.
These changes do not depend on external conditions for their origin. Such conditions may hinder or arrest their development for cannot cause them. Germinal variations may occur at any period of life from the beginning of the life to the death of the organism. Examples are: occasional occurrence of supernumery digits in man, domestic cat, horse and other animals.
2. Somatogenic or Somatic Variations
The variations in the soma and the body cells caused by the factors of environment such as food, heat intensity of light, pressure etc, are called somatogenic variations. These are also called the acquired variations as they are acquired by the individual during its life-time.
They are lost with the death of the individual. They are neither inherited from the parent nor transmitted to the future generation.
For examples, the long neck of giraffe, well developed muscles of an athlete, sometimes there is loss of digit in men etc.
3. Continuous Variations
If the variations occurred very small, abundant and in a graded series then such variations are called continuous variations. These are usually qualitative. If these variations occurred accidently (fortutious) they are called Darwanian variations, or fluctuations which are acted upon by natural selection.
Examples of continuous variations are: gradation in stature, height and intelligence in man; skin colour, weight and milk production in cow etc.
4. Discontinuous Variations
These variations appear suddenly without any known reasons. They show wide deviations from the normal condition and are not linked with integrades. These discontinuous variations are also called saltations, or sport or mutations.
For example: in vertebrates often 6, 7 or even 8 digits have been observed. Such variations may also occur in the segments of worms; vertebral column, ribs, muscles and in the number of appendages. These variations are quantitative.
5. Indeterminate Variations
Variations which are not governed by any law but take place in some imaginary or thinkable direction of change. Thus these variations depend on chance only variation of size in closely related organisms is an example of indeterminate variation.
6. Determinate Variations
These variations are governed by some unknown force and are imposed to occur in one line or certain lines, usually in an adaptive direction. These are also known as orthogenetic variations. This type of variation is seen in the evolution of overgrown anthers of Irish deer or tusks of Jefferson mammoth.
Besides the above variations these are numerical or meristic variations.
When an organism possesses additional similar part than the normal number, it is said to have undergone meristic variation. The development of an extra digit on the hand or foot of a man, the occurrence of six instead of the normal five arms in a starfish.